Improvement #1
Eliminate the input lag in RTTP. NBA 2K sells close to as many copies each year as Madden and FIFA, yet those two games play flawlessly online, while NBA 2K continues to be a lag-fest online.
Improvement #2
Create an alternate RTTP ladder with the following lineup restrictions:
A) 2 gold players OR 1 sapphire/diamond/ruby/emerald player
B) 3 silver players
C) 7 bronze players OR 8 bronze players if you picked a sapphire/etc.
In addition to that, you would have a team salary that's based on all your cards' combined overall ratings. This would prevent everyone from picking LeBron/Durant/Jordan as their two golds. People could only afford to pay one of those "GOAT tier" players, and would have to settle for a lesser #2 gold guy instead like Wade/Westbrook/Pippen.
Basically, they just need to make RTTP like All-Pro Football 2K8, which to this day, is still one of the most balanced multiplayer sports games ever made.
The mode would also be way better if you had access to every player in the game from the very beginning, like in APF 2K8, and didn't have to go through all of this card collecting nonsense.