So...I'm going to take a stab at actually designing the "normal person with no magical ability that just uses trinkets and stuff" that Grefter was talking about. I'm trying to follow the design and balance of other classes--for instance, I've noticed in the book that if a class has an ASI, that will usually be literally the only thing they gain on that level. Exceptions seem to be made for minor stuff like cantrips known. For some reason, Cleric is a big exception to this, gaining a fairly significant domain feature on 8, but nevermind that.
This is also going to be a weird class to balance, because there's no telling if your DM will actually give you magic items that cast magic, especially if they just roll on a loot table. So...it's going to need goodies so that you feel reasonably valuable even if those class abilities aren't active.
Tinkerer
You are a master with your hands, and a clever problem solver. If there is a tool that exists, there's a good chance you can figure it out. You are trying to crack magic as well, and are certain there must be a logical scientific explanation for it all, but for the moment it baffles you.
Hit Dice 1d10 per Tinkerer level.
Proficiencies
Armor: Light Armor and Shields
Weapons: All simple and martial weapons
Tools: Choose any three
Saving Throws: Dexterity and Intelligence
Skills: Choose any two
1st level
Jack of all Trades
You can add half your proficiency bonus (rounded down) to all skill checks that don't already use your proficiency bonus.
Nimble Hands
You can use your bonus action to take the Help action, do the search action with intelligence (Investigation) or wisdom (Perception), take the Use an Object action, Make a dexterity (sleight of hand) check, Make a wisdom (Medicine) check to stabilize a creature, or make a wisdom (Survival) check to identify natural hazards like quicksand.
(Note: no the Use an Object action does not let you activate a magical device).
Clever device usage
Instead of using the DC of a magic item, you may use your Tinkerer spell save DC (8+INT+prof mod).
2nd level
Notebook of Magic
Magic still defies scientific explanation for you, but you are taking notes and have begun to make a little headway. Choose a cantrip from any class to copy into your notebook. You can cast this cantrip, but only if you are holding your notebook in your hand and only if you are currently able to read it. These count as tinkerer spells for you and use your spell save DC (8+Int+prof mod). You may copy an additional cantrip into your notebook at 4th, 6th, 8th, 10th, 12th, 14th, 16th, and 19th level.
Use Magic Device
You have figured out enough about the working of magic that you can improvise the use of devices that were not built for you; you ignore all class and race descriptions on items.
Expertise
Choose two skill checks or tools to get double proficiency with.
3rd level
Extendable Arms
You can spend 50gp and one long rest to craft extendable arms. Using a bonus action, you can extend or contract the arms. While your arms are extended, your reach is increased by +5, but if you try to attack a creature within 5 feet of you, you attack with disadvantage. While your arms are contracted, you attack as normal. Extendable Arms can only be used by Tinkerers of 2nd level or higher, or characters with the Use Magic Device class skill.
Ritual Casting
You can now copy ritual spells from any class into your notebook of magic, and you can cast them only as rituals, and only if you have your notebook of magic. The cost of copying spells into this book follows the Wizard rules. You can only copy a ritual spell into your notebook if it has the (ritual) tag, and the spell level is no greater than half your Tinkerer level (rounded up). Additionally, you may add one level 1 ritual for free when you gain this feature.
Item Metamagic (Subtle Spell)
Your tinkering with magic items has given you a skillful amount of control over the spell that comes out. You can use magic items as if you used the Subtle Spell metamagic. You may only use one metamagic when you activate an item.
4th level
ASI
5th level
Extra Attack
Blah blah blah hit things.
Item Metamagic (Careful Spell)
Your expertise with magic items has allowed you to sculpt the spells they produce to reduce harm to your allies.
Bonus proficiencies
You gain proficiency with 2 more skill checks and one tool check of your choice.
6th level
ASI
7th level
Expertise
Choose two more skills or tools to add double your proficiency.
Battlemagic
You've mastered weaving magic and weaponry together. When you cast a spell or use an item to cast a spell, you can use your bonus action to make one weapon attack.
Item Metamagic (Empowered Spell)
When you roll damage for a spell, if you use this metamagic you can reroll a number of damage dice up to your intelligence modifier.
NOTE: unlike the sorcerer version of this metamagic, you can not combine this with other metamagic.
Spring Mounted Pillow
You can spend a long rest and 50gp to craft a spring mounted pillow. While equipped, when an attacker you can see hits you with an attack, you can use your reaction to press a button extending the pillow to halve that damage. This can only be equipped by Tinkerers of 2nd level or higher, or by characters with the Use Magic Device class feature. If someone -without- proficiency equips this, they can successfully press the button to block one attack, but they won't get the pillow back into its compartment, causing them to make attack rolls, dexterity based skill checks, and concentration checks with disadvantage.
8th level
ASI
9th level
Helicopter Hat
You can spend 100 gp and a long rest crafting a device that is a hat, with a helicopter that can sprout from inside of it. It allows you to at will cast Levitate on yourself. If the hat is removed from your person, the effects of the spell end. Operating this Helicopter Hat is so complex that only a Tinkerer of 2nd level or greater, or someone with Use Magic Device can use it.
Item Metamagic (Extend Spell)
You can extend the duration of a spell cast by an item to twice its normal length.
10th level
Spell Empowerment
When you use a damage or healing spell (through an item or through innate spellcasting ability) you can add your Int modifier to one damage roll, or healing roll of the spell.
Jack of Your Own Trades
You now add an extra half your proficiency bonus (rounded down) to all skill checks, not just the ones with which you lacked proficiency. (So untrained -> 1/2 prof. Proficiency -> 3/2 prof. Expertise -> 5/2 prof).
11th level
Item Metamagic (Quickened Spell)
You can cast spells from your items as a bonus action when you use this metamagic. Note that normal spellcasting rules apply (you can't cast a bonus action spell and a non-cantrip in the same turn).
Ze Goggles
You are able to spend 100 gp and a long rest to manufacture goggles that allow you to see in magical and nonmagical darkness as if it is bright light, see invisible creatures, and see through illusions, but limits your vision to 30 feet while you are wearing them (you are blind beyond this distance until you remove the goggles). These goggles also have the side-effect of making your eyes appear twice their normal size. Operating these goggles is so complex that only a Tinkerer of 2nd level or greater, or someone with Use Magic Device can use them.
12th level
ASI
13th level
Elusive
You are so adept on your feet that attackers rarely get the upper hand on you. No attack against you has advantage while you aren't incapacitated.
Metamagic (Heightened Spell)
When you make a spell that requires a saving throw, you can cause one target of the spell to make their saving throw with disadvantage.
14th level
ASI
15th level
Item Metamagic (Twin Spell)
When you use an item to cast a spell that doesn't have a range of self, and is capable of targeting only one target, you can choose a second target.
Evasion
When you are subjected to an effect that lets you make a dexterity saving throw for half damage, you instead take no damage on a successful save, and half damage on an unsuccessful save.
16th level
ASI
17th level
Item Metamagic (Distant Spell)
You can double the range of spells you cast using items. If the spell has a range of touch, it now has a range of 30 feet.
Extra Attack (2)
You can now make three attacks, just like a fighter.
18th level
Superior Helicopter Hat control
You have become so adept with your helicopter hat that you gain the following benefits while using it. You can replicate the effects of the Fly spell on yourself, and staying airborne no longer requires your concentration.
19th level
ASI
20th level
EUREKA, Magic makes sense
Your SCIENCE is finally starting to make a breakthrough in understanding magic. Add six 1st level Wizard spells into your notebook of magic. You can cast them without using spell slots as long as you are holding your notebook of magic, and as long as you are able to read your notebook. You can copy 1st level Wizard into your notebook using the normal Wizard rules for transcribing magic.
(EDITED)(Commentary: this capstone is pretty broken. Infinite out of combat healing. Spam Bless as you walk around. Use the Shield spell in-combat if you have a hand free to hold your notebook--basically turns your notebook into a +3 shield. But it's a capstone, and most of the benefits are out of combat, so whatever).
Mmm...ok, so balance concerns (other than yes the capstone is busted)--does this crowd out Rogue? I don't think so--Rogue gets most of the same abilities, a bunch of extra ones I threw out due to ASIs on those levels, and Sneak Attack, and honestly Cunning Action is probably better than the bonus action options I listed here (which is why I put that as a level 1 ability). The one ability Rogue doesn't get is advantage on initiative (I took that from Barbarian) and Jack of all Trades (taken from Bard). Does this crowd out Fighter? Maybe? Well...ok I mean...no medium or heavy armor, no fighting style, no action surge till level 18, no indomitable. No archetype bonuses like superiority dice. TBH fighters should deal more damage at basically every level. But Tinkerer gets the same ASIs combined with some Rogue goodies like Evasion and Uncanny Dodge and skillmonkeying. Maybe I should remove one of the ASIs? (Fighter gets 3 extra ASIs, Rogue gets 1 extra, maybe Tinkerer should only get 2 extra? Remove the ASI at...level 10? Mmm...maybe; "dip 11 levels of fighter for a 3rd attack" is a thing--Tinkerer does force you to dip to level 13 for that attack, but maybe it should also give you one less ASI for your trouble? Although...honestly, the kind of multiclass that takes 11 levels of fighter to optimize weapon damage probably wants a fighting style, and probably wants action surge, and might very well want heavy armor. So...maybe there's no issue?)
