Yeah a reason to play solo after beating the mission would be nice. They always reply that this game is meant to be co-op to pretty much any solo mode talk, but these suggestions give solo a reason to keep existing without being a suggestion that would overpower the online play.
Something to jump into every so often, which I believe would encourage players to improve their skill level without the crutch of better players carrying or defending them, and giving occasional rewards outside of the current model. When I first played, I thought Very Hard was totally impossible, but I've cleared 4 Very Hard solos so far, actually did Hangin' By a Thread without a single life lost. It teaches optimal character choice and play style to deal with the objectives presented.