As the game keeps growing and the character base keeps expanding maybe roster slots should start being something that is obtainable through playing the game; maybe make them a rarer prize to be sure, something you can only get through legendary tokens;another thin that might help in this area could be switching the slot cost from 1000 hp at a certain point to 1000 hp OR 25 command points (something like that).
Just an idea that popped in my head! I figure something like this can help the longer term players keep growing their rosters in a competitive fashion