So I didn't get started yet because we had a familly dinner, but had some ideas floating around for some specific questions (you know the interesting ones).
These are straight copies of notes from my phone (late, tired, lazy), but hey food for thought/discussion. [Used for extra thoughts outside of notes]
Magic concepts
Churning abilities to new colours
White sacrifice. [I need to research how much it has been used for White, I generally think of sac as a Black mechanic though]
Green Draw Card
Black counter (COST!)
Red colour change (Controlled?) [I don't really like this one, but the Lace line of spells are criminially underutilised components of red]
Blue First Strike/Flanking (Check existing?) Not great
[I know Blue has a hint of this back in 4th with First Strike, illusions generally being hard hitting first strikers that are weak was flirted with, but I think it generally has been dropped to the wayside with Red/White predominantly being First Strikers. Might just be more of a revisit. Flanking is a pet favourite mechanic that I tend to think is underutilised, to overly tuned it can be crippling to the enemy, but I like it because it restricts it entirely to creature control while retaining general weakness of the creature vs players and doesn't buff the card amazingly against burns]
[Rambling about why I think Ravnica is an awesome set]
Guilds are great, mix of colour encouraging themes (player type, Timmy?) needed more emphasis on focus, easy to ignore core mechanics, not enough guild cards (limitation of set mechanic [monetary requirement, cannot release one huge set to last for 6 months anymore]. Hard balance to maintain limits usability in extended/unlimited (No worse than Shadow).
That is all I had on the bus, but hey it is the start of a concept of an answer for the most interesting question and a pretty honest start to one of the other questions.
Other ideas my brothers threw out, elder brother said pingers for Black, touches a bit close to ground it already covers I think. Little brother said anything that gets rid of Counter from Blue because he fucking hates blue. Hater got to hate I guess.
Personal favourite of the ideas that spat out, is White Sacrifice, I would like to see it in a White is Not Nice set. Even have a Rare concept I would like to float, just need to hammer down a cost. Like I have noted though I need to really research how much White Sacrifice mechanics there already is (I know there is some basic ones for destroying enchantments and some that gain life/prevent damage), but I really would like to see it roll out as a more primary mechanic instead of just being a late game booster to pricey 2/2 summons.
One I would probably run though honestly is the Green draw card one. Green hasn't done this before, green searches or mills itself to find what it is after. I want this in the same set as White is Not Nice honestly, I would like to see a nurturing Green. Green is always ferocious because it is you know, busy trying not to be elvish vagina monsters all the time or WARRIOR ELVES. I would like to see green reworked once to be a more spell oriented druidic based concept for a set, growing and germinating new ideas instead of being Survival of the Fittest type stuff. Dovetails really badly with existing sets sadly, but again, no worse than Shadow does.
Fuck Shadow and it ever existing, fail twist on Flying. I thought about trying to work out a way you could revive Shadow and work it, but short of devoting an entire set to it I honestly don't think it can happen (which is part of why Shadow sucks, the Rath Cycle had so many plates spinning that Shadow should have been taken out the back and shot as it had no room to flourish, other places it was brought back just make it worse), but again this new set would dovetail HORRIBLY with other sets in Extended or Unlimited.
But anyway I have babbled more than I meant to.
Necropotence is awesome design and horrible design. It is a fun concept and it is hella powerful and has the cost to leverage it (BBB honestly is I think the power level I would balance my conceptual White Rare to WWW is expensive and perfect) Edit - Note I mean I like this as a flavourful cost much more than power level value. What is wrong with it though met?
If the card has to use 8 (maybe 6/7?) point font on the card just to fit it then there is a good chance the card is over engineered. It falls flat in one of the core things they say somewhere in the entry. It is just to complex. Some of it can be stripped back with more modern less rulesy effect writing, but still, it is a pretty nuts card.