Give teams an option to stretch out a prorated signing bonus for more years if a player is released. If that's too much, then make it for every reason other than performance. If a guy keeps getting arrested or is injured or whatever, let the teams spread that money out a bit. I know the player's union would be against this because high cut costs keeps under contract. But it would be better for everyone if teams weren't stuck with a ton of dead money for something they couldn't predict.
Along those lines, remove the rule preventing teams from paying part of the salary of an outgoing player in a trade. The league already sort of lets this happen with the bonus conversions for guys like Clowney or Eugene Monroe back in the day, but get rid of the line in the book so teams don't have to skirt around it. Let teams sell cap space. If you don't change the proration rules for trades, then that is still a pretty big disincentive to sign a guy to a big deal just to try to trade him later. Or , limit it to a percentage of the player's salary or whatever. But I don't think there's anything wrong with selling cap space in a league with a hard cap and no salary matching.
I'd also like to see them raise the compensation for practice-squad players and make them more like two-way guys in the NBA are. A team has the option to sign X many guys to exclusive practice-squad deals with guaranteed money but also no chance of the player being poached. That player can then be called up just like in the NBA, and the team doesn't have to make a corresponding roster move like they do now. When called up, that player gets the NFL min for however many weeks they are on the roster. You can balance it out by not letting players sign those contracts if they are draft picks unless they are first released and put on waivers. But doing this gives guys at the bottom of the roster more money and allows teams to have way more flexibility than they currently do. You could have something like a four-week limit before the contract has to be converted too. Just like the PS deals in this scenario, the converted contracts would be guaranteed for the season to prevent abuse.
I'd like to see the IR system continue to evolve. A short-term disabled list makes a lot of sense in a league where guys get hurt all the time. Being designated to return is a nice step, but really if a guy is going to be out like four weeks or whatever, there should be an option to replace that player. If you combine this with the two-way practice squad, you can limit the ways teams could abuse this system. You couldn't use it to keep a bunch of guys, because you have to expose everyone to waivers if you put them on the practice squad. And if you could only use guys from this practice-squad pool to supplement guys on the DL, then you aren't actually adding more players than you've already signed. If you need to actually sign a guy to replace an injured player, then that player has to go on season-ending IR to get the roster spot back.