Well I hadn't played FM for ages, but got back into it through playing Fifa Manager 07, only played it a few days because, essentially, its **** compared to FM07, but it had nice features that got me interested in football managing games again, like viewing the stadium, designing new ones, assigning ticket prices, buying up land around the stadium and building things.
The most important thing for long term playability is the regens, to model their stats on real life players, I don't mean go back to the regen system just some logic to the stats, there's a huge database of real life players which have been scouted showing the kind of stats players who have made it have. The fact is the temperamental superstar in real life is rare, after 25 years in FM its the norm (I have the best squad in the world by some way but only have one model professional), its also too easy for players to fulfil their potential, mental attributes only seem to affect how they get along with other players and how they react to being left out, bids being rejected etc.
I would suggest changing the youth system so players come in from the age of 8 or so, you could get loads of players that age either with set stats or variable ones as things like determination etc wouldn't be set at that age, by the age of 15/16 most would have dropped out, those with poor mental stats and you would bee left with those who have great mental stats and are more certain to reach their potential, whatever that may be, and those who have great talent but poor mental stats who are less certain to reach their potential. This would obviously be affected by youth set up, coaches, scouts, affiliated clubs and so on.