By reasonable I meant that it would be acceptable given the price per character of almost every current fighter. Honestly, I hope you're right but I think you're going to be disappointed with the Gamescom announcement.
Also, a 20+ roster seems unlikely and unnecessary, especially for KI. Once you start getting above 16, you either have bundles of characters that are much too similar (SF) or a roster that's very hard to keep balanced (MvC3). Personally, I'd much rather have 8 unique, perfectly balanced characters VS. 5 characters that are overpowered, 10 characters that rely on specific match-ups, and 5 completely worthless characters that nobody ever uses.
An 8-man roster seems tiny but we're all going to pick one or two characters to main anyways, no? The slow trickle of new characters -- and balance adjustments that'll likely come with them -- will keep the game interesting for a lot longer, IMO.