Things I would suggest you look at immediately for inheritance for any generic build (i.e. not Horse Emblem or whatever):
Assist: mobility (Swap/Reposition/Draw Back, possibly Pivot/Smite) or Rally Speed/Attack (+others if running a *blade mage). Swap feels easiest to use as default, off-hand (does not necessitate any terrain around the units involved to work.)
Special: Moonbow. Really hard to justify anything else as better due to 2 charge. Reprisal as another 2 charge option on a build you expect to build limit damage on, but only learnable from 4* Kagero or 5* Karel at the moment (and if you pull the latter, you better be transferring Wo Dao+ or you're crazy). Moonbow is even better if you're running a Killer weapon since 2->1 is huge and better than 3->2. If you want upfront damage, with the meta trending towards glass cannon units, one of the 3 charge supers that uses your own stat as a damage base is worth considering as well.
Luna and Vengeance, the 3 charge versions of these specials, are still perfectly respectable if the unit already has them of course (probably don't need to teach Moonbow to Lucina).
A: Probably the most flexible slot. Speed + as a generic option, preferably on chars with high speed already to solidify them. Darting Blow, Death Blow, Life & Death are all offense oriented options. Fury is interesting; even with it not factoring into BST it could work on some units, especially those with sustain (Falchion/Renewal). Distant/Close Counter are also here.
B: Vantage of course, to the dismay of many Lon'qus who have lost their souls since Thursday. But also some other interesting stuff worth taking a look: Wings of Mercy (armored and dancer units especially), *breaker skills, Desperation for more frue glass (stir with Ardent Sacrifice as a reliable way to activate it). The Seal series also lives here and while I don't respect stuff that activates out of combat that much, if you've got a unit with a lot of bulk Seal Spd is worth looking into.
C: Echoing NEB that Threaten isn't especially good, although it's fine on armored units by the nature of how they work. Hone/Spur are the usual suspects, but a surprise filler slot that can work is Renewal, especially with Fury as mentioned above (only 4* source right now is Fae, though).