Erm... I think you need some more experience playing this :)
Those expensive players are actually salary efficient players, you don't need money to train them. Top level yes, but you can still train some mid-range salary guys whit little to no investment. I'm currently training an allstar potential guy, he's going to hit some 110 skills with 25k salary. I trained with a cheap lvl 3 trainers and got him as a mediocre draftee, you can find those for 1k on the transfer list.
So basically, salary efficient(regardless of salary) will go high on the transfer market, salary inefficient(regardless of salary), is crap, and can be sold for a bit of cash.
Also, regarding your suggestions:
1. We have over-extension tax implemented, it is a salary cap of sorts.
2. Free agency, where you actually get players for free? Maybe, but that would need a complete restructuring of the current system, maybe adding player contracts and stuff..
3. Trading is definite no. Never. Ever.