For what it's worth... I think FE10 Elincia would be an interesting addition. Has an interesting quirk (Amiti multi-hits so defense is important against her), a weakness (bows / wind), options in damage vs. healing, and Authority. I'd gutcheck a 2.0 to a 2.5? Less raw broken damage than Ephraim, less dodging than Lyn, and arrow weakness comes up a lot in the Dungeon, but Authority & healing are pretty cool to make up for it. She's also vaguely Marcus / Jagen-like for cheesing Floors 1 & 2.
The one bit of slight wackiness is that Elincia isn't on the team for Chapter 3, the longest chapter in the game, so it requires a bit of squinting for that period. Also, I believe Meeple's stat topic slightly undersells endgame Elincia, since she's level 10 there... but she's a pre-promoted to tier 3 unit, so that means she gained a mere 9 levels in-game. I'm actually not wholly on board with equal-XP, but Elincia & Sanaki (& Bastian & Volke) deserve a level bonus no matter what since having them end the game at level 10 implies you barely used them. (Elincia is lvl. 19 vs. a lvl. 12 average in Dark Holy Elf's equal-XP list, and would probably be lvl. 19 vs. a lvl. 14 or so average for me since I'd let the Dawn Brigade members catch up to the level 10 stat topic minimum.)
Rough outline:
Floor 1, 2-
Slim Sword, Heal Staff (Elincia is grotesquely over-statted as a tier 3 unit but her damage is still meh thanks to the bad sword. Good luck hitting her or dealing damage, though, and she doubles everything.)
Floor 3 -
Amiti, Wind Edge, Mend Staff (2-E, gets access to a shop. Uber offense with super-speed and damage, but can now actually die.)
Floor 4 - As F3, but less so. (Alternatively, Unarmed Guest for whom it is game over if she dies form?!!!)
Floor 5 -
Recover Staff, Restore Staff. Let's squint and stick 4-2 as floor 5 worthy. These are technically for sale in Chapter 3, but Chapter 3 is big enough to extend into Floor 5, and Elincia isn't there to receive the purchase, so questionable legality for F4. (Not that Elincia ever cares about the Recover Staff unless her magic stat has been hugely debuffed or something, Mend is full-healing even for the tanky.) Any Elincia starts gaining levels here so transitions into wherever you see endgame Elincia.
As for the Authority stars... what does +15 Hit and +15 Avoid do? Well, see
http://serenesforest.net/general/truehit.htmlThe +15 Hit is pretty good so long as you have at least a 50% chance of hitting. A 50% hit rate becomes a 76% hit rate, a 70% hit rate becomes 89%, and a 90% hit rate becomes 99%. In the same way, the +15 Avoid helps the most when you're already a bit dodgy. A character from a game with no evade against a vanilla attack has the 100% hit rate become a 95% hit rate thanks to Elincia, hardly an inspiring boost. However, even 10% dodge (77 Fire Emblem Hit) becomes 29% dodge (the reverse of the 70% Hit -89% hit case). A character like Yuna, who has 68% physical evade (=40 Fe Hit), sees that jump to 87% evade (=25 FE hit) and has 5% magic evade tacked on.
Another minor bit of wonkiness is what to do in case of Elincia death. Obviously this gets finnessed in the dungeon since Lyn / Ephraim deaths are always game over, too. The death of any FE10 commander is always game over for you, but there are some fights where defeating an enemy boss with authority stars doesn't end the battle, and sure enough enemy team members lose the Authority Star bonus in that case. I'd definitely deny the authority bonus while Elincia is dead, and quite possibly throw it away for the rest of the battle if Elincia ever dies just on flavor grounds, since zombie commanders isn't an issue that ever comes up in FE10. Would be willing to be convinced one way or the other, and doubt it would matter anyway. Since Authority Stars don't stack and only the "leader"'s stars are relevant, I'd definitely argue that any other character that requires an equivalent "leader" position is out of luck. (e.g. pick one: FF13 Summons -or- Elincia's Authority.)
As for if Hit / Avoid matter against status like Sleep Staves... uh... I have no idea. (It'll certainly help against Venin Swords, but that's an evadable physical with 100% poison attached obviously). Dark Holy Elf? Anyone?
