I can't agree on any cap, badge or otherwise.
If you want something, you can have it if you're willing to work or pay for it. If a PG wants the gold tenacious rebounder badge, then go out and earn it. 3-5 offseason training sessions each year, each badge can be upgraded once a year.
Once you earn the badge, you can compete in rebounding drills against the 5-10 best rebounders in the NBA during the offseason and if you somehow manage to get the better of them that unlocks to ability to purchase the next level of the badge. A 6'3" PG is gonna have a tougher time than a 6'10" PF competing in these drills.
You can purchase extra training sessions or buy the next level for a badge outright with VC, one skip per badge per offseason and it still takes up a training session.
The NBA offseason is roughly 19 weeks long, each training session can be about 1.5-3 weeks long, going up against 1-2 opponents a day. This adds some kind of importance to the offseason or you can skip the whole process if you choose and miss out on the upgrades.
Again, this adds playlife to the mode and limits the ability to just purchase a ton of VC and be maxed out with all of the badges by the end of the first week. You have to pay and work to get your player to get to 99-badged-out-god-status. This all lends itself really well to connected-careers if 2k goes in that direction.