I'm not sure if this has been suggested before (i'm still a fairly new BB forum troll!).
What i'm proposing is that enthusiasm be player based rather than team based. For example today i played it easy and played my weaker players, as it was a match where the outcome didn't matter and i wanted to get my enthusiasm up for the playoffs. However I think teams should be restricted in this way and made to play their better players during an easy game to gain the resulting enthusiasm. This will hopefully lead to more competitive games towards the end of the season, and perhaps balance things heading into the playoffs a bit better (it seems weaker teams arrive to the playoffs with a few more injured starters, top teams sort of get "bonus" training weeks currently, where they can play their trainee centers at point guard etc). It also may reduce/increase the value of players sold near playoff time.
I know this is somewhat already in place with game shape, but i believe it might add an extra dimention for GMs to think about. As long as enthusiasm decays as it does over time, you couldn't "charge" your best players for long periods of time either.
As it a simple yes/no (26 mins+?) it shouldn't have an bearing on the game engine and probably wouldn't be too hard to implement.
Any thoughts? (And thanks for reading!)