I think it'd be a good idea to make players who perform well demand more money. Right now, you can resign low rated players for cheap, even when they heavily outperform their ratings. Sure it could be the system that they're in, but nobody in real life would be willing to accept chump change after they've thrown 60 touchdowns or have run for 2000 yards.

Also don't allow for 6 year minimum salary slave labor contracts. I for one take advantage of this but it's also highly unrealistic. Just because nobody has offered these free agents a contract doesn't mean they should cave into unrealistic demands. You can also resign guys for pretty cheap if you just spread them out over 6 years. I am currently paying a near max OL just 4m a year and he's basically my most expensive player that I have through resigning.

I would consider adjusting the player's bonus demand to something much higher. The player would then have a hidden adjustment between 0% and 50% lower bonus that they are willing to accept. If the owner offers a contract that is not within 10% of the hidden adjustment (lowball), the player will then refuse to resign or raise their salary demands. You could also factor in player opinion into how high their hidden adjustment would be. This would make resigning players a bit more interesting and force owners to open up their wallets or else the player may refuse to sign or demand more money.

If we're going to do this, then draft pick contracts would need to be fixed as well. Currently 2-3rd rounders make way too much, even those guys at the end of the first round are getting overpaid.

I have actually thought about this, but my concern is that it could be manipulated by an owner who needs to renegotiate a star player and so just makes sure he doesn't get a chance to get good stats. So I try to make the requests based on objective attribute values instead.

Is there a way to combine the two?

I previously played in a game similar to this, and that is exactly what owners did. But we came to accept that it was part of the game, and that if the owner was willing to throw away a season or two of that players career for a cheaper salary, then so be it. To combat your concern, you could do a check to see which one is higher, a players rating based salary demands or a players stat based salary demands. The higher of the two will be the salary that the player demands. That way, if an owner benches a star player, that player will still demand a lot of money while an overachieving player will also demand a lot of money for performing well.