First of all, I really think you brought up some good ideas. I'm just sceptic about implementing them.
The season updates of the salaries would have a huge effect, that's for sure. I don't know if you had checked the staff members prices when Utopia started but it was crazy. Imagine that with 10 times more managers. Nobody would buy staff members in the last weeks (before salary raise) but almost everybody would like to buy in the first weeks.
Experience would really be nice and I totally support that idea.
Skill level is a difficult question. If it is an integer you could only raise their level with one major change and it would (or at least should) cost you huge money, otherwise the game will be full of level 6-7 trainers with 15-30k salaries in a couple of weeks. If the staff member skill is a decimal number you could slowly raise it to the next level which is better but also have the dangers. Having more high skills trainers could destroy the whole economy module because everybody would start training players that they can't afford. I would look for some alternative ways like I mentioned some. Or maybe new "special" skills, like offensive/defensive/athletic trainers that would speed up training in some areas while slowing it down with others.
Also I think that all of your ideas at the same time are too much (having a cheaper trainer, not raising his salary, evolve him AND earning experience). I prefer the experience only and maybe the evolve but some other way.
Don't feed the troll