Nice idea for a PT and one that I actually had to think about for a bit. One of the first things that came to mind for me were stats like Experience and Leadership who already exist but don't actually have an effect in the sim at the moment. But those aren't really new stats so I don't think they would count, so I went for something that could be seen as a bit of an extension of those Stats: Mental Attributes. Those attributes wouldn't necessarily all be ones that you actively improve either, but ones that work through a slider were being towards either end of it comes with different positives and negatives. Think of aggressiveness for example, where a high value might give your team a boost once in a while because you inspire them, but you also pick up more penalties. Or something like flair where you can build a super flashy player who has some great moves, but which might also lead to him becoming cocky or less consistent.

Maybe even more important than those new attributes would be new ways to work on these attributes aside from your regular old TPE-purchases. I already talked about sliders where you could pick to have certain attributes at a value between 1-7 at the start of your career, and then would have to invest money later if you wanted to make (gradual) changes. Another idea would be attributes or perks that you develop of the course of you career based on your achievements. Being a loyal player for your team could give you a bonus in certain attributes as could being a high-paid FA, being a proven winner or being especially hungry because you haven't won anything yet could give you bonuses but maybe also a small malus in other areas. You could also tie this to forum achievements or general activity, so that people who post a lot or do on-site work can gain specific perks for it. Maybe make it so that those perks aren't tied to performance-based stats because it would give an unfair disadvantage to people who are active but just quiet, but create a new attribute like "marketability".

Players with a high value in that attribute are ones who the fans and sponsors love (and forum activity wouldn't be the only way to earn points), and those players maybe give the team they are on a financial bonus through jersey sales and new sponsors and what not, and maybe the can also command more mone in negotiations and through sponsorships. This could be achieved through a modifier that people build by raising their marketability. The skill starts at 0, where no bonus is given, and every 10 points they earn a x0.1 bonus, so x1.1 at 10 all the way up to x2 at 100. This modifier could then be used in various ways, for example in contract negotiations. The player would still count towards the cap as he does now and contract minimums would still apply, but a player who is paid $5M by his team and counts against the cap for that amount would actually receive $10M. We would probably need a way for this value to degrade over time or maybe even make it a usable ressource, so every time marketability is used to get more out of a contract, it drops by 25 points. Or make it something that you can use to earn a "free" 2x bonus for a media article, but for every extra 1M you make through it, marketability drops by 10. It could be a neat little way to reward people for forum activity and other activites that are good for the site and give an extra incentive to engage in those things, but have the rewards purely money-based in order to not create TPE-inflation or allow the super actives to pull away further.