There were indeed a few enemies that did require a Mimic setup for me to clear properly. I don't think it's too cheap considering the setup required to pull it off, though. One of the last ones I did (one of the Feral Chaos') involved having Bartz mimic Spiritus' unique ability. If you set up the rest of the party to be boss nukes (or Yuna Grand Summoning at the beginning to clear out an early wave) you should be able to guarantee that Bartz mimics that and then boosts Shantotto's damage 3.4 times without too much luck.Congrats! I'm currently at 98%, only eight quests left (most of which involve beating (Feral) Chaos), so hopefully should hit 100% soon too, close to getting all characters to level 99 as well. I'm curious, did you ever have to rely on Mimic to beat any of the quests? I see it's supposedly very powerful but I've not been using it at all since it seems both a bit cheap and also a pain to set up properly.
I'd love to get some DLC characters too, but I think it seems unlikely given only FF characters are in the game and the DLC is all songs from other series, unless the characters are released separately. At least the World of Final Fantasy and Stranger In Paradise City protagonists should have been added, since there are songs from both in the game. But Wakka, Lulu and Kimahri are the biggest omissions for me.
Some other Mimic builds can get a bit meme-y, and you can most likely brute force most quests instead at a certain point by just running an Ace/Aphmau/Lightning #2/Spritus (+/- Shantotto somewhere in there) party once you start gaining Star Levels and blowing your stat boosters.
The dumbest party I had to build was to beat Cloud of Darkness and involved using Guy's unique, but I had to purposefully not get criticals for about 90 notes so it would trigger at the right time.
And yeah, that was exactly what I was getting at re: DLC. I think there are still some omissions in existing Final Fantasy songs (and full versions of ones already represented), and I wouldn't be surprised if we do get some paid DLC in the form of a second season pass to add some of those, FF16 music, and some missing characters.