This is how adding combo quips currently looks for me in the backend.
I currently have uniques all the way up to 150 (there's a couple gaps in there, but they're easily fillable), and was planning to have uniques up to 250 (even though I don't think they'll be visible much if at all in live builds, but you never know), after which it'd move in intervals of 10, 25, 50 and 100, as it went further down the line, with some significant numbers here and there thrown in for flavour, and cap it at 999 (Which is what Skullgirls also caps at, last I knew)
So, adding more combo quips?, doable.
Adding character-specific combo quips?... IIIIIII don't think I really want to go there, to be honest. One of those things that I'd probably take a look at and ask programmers to do near the end of the project if there's time and it's relatively easy, but... you know, not really near the top of my flair & fluff list.