I don't know if this has been suggested before but I would love it if some sort of team chemistry factor were introduced. Not necessarily some rating we can see as managers but just some sort of hidden variable that would reward teams for keeping players together over a long time. It would also discourage day trading and make team building fun and involving. Games would be a tad bit more unpredictable too. I think it would add a great new dimension to this game.
It is an interesting thought, but it has its flaws.
To build up "chemistry" in RL is certainly much more complicated than just letting the same players play together for a long time. Actually if you keep the same team without any shanges for several years the player will probably start to get lazy, loose inspiration, and chemistry will start to go down as players start to get tired of each other. Thus, just saying that having the same roster for a long enough time would increase chemistry is not correct.
Furthermore if focusing of the game this would benefit inactive teams rather than active teams, and I'm sure that inactivity is not good for the game longterm.
Therefore if chemistry would be introduced it should be more refined than waht you suggest. It should emulate real life effects while at the same benefitting the kind of usage that is likely to be benefitial for the game on longterm (i.e. the right "level and kind of activity" by users).
I'm not sure how to accomplish this though.