I can relate to you on the monotony of Summer League in previous editions of 2K. Some additions I'd recommend to enhance the draw and replay value of Summer League and D-League:
1. Include the Ability to Simulate Games (or skip Summer League altogether): I believe in a simulation Summer League/D-League schedule, but I also believe in giving gamers choices. I could see myself getting to a point where I wouldn't want to play a full slate of games while still loving that I
can play in the Summer League or D-League if I wanted to.
2. Authenticate the Summer League/D-League environment: Currently, there's two Summer Leagues: one held on the Orlando Magic's practice court (lasting one week) and another held at UNLV's Thomas & Mack Center in Las Vegas (also one week). I think having the proper areas hosting these events would do a lot for the sake of immersion and authenticity.
For the D-League, I'd understand authentic arenas would be tough to come by (a little more difficult catering to an entire league's arena lineup versus the Summer League's mere two venues). Still, I'd at least like to see the proper court art for each squad.
3. Proper Summer League Gear: This is largely in reference to employing proper practice jerseys for summer league in addition to including proper D-League jerseys. NBA 2K offers practice gear as wearable options already. It'd be nice to give them a real spot to be worn (aside from Practice mode).
4. Assistant Coaches Heading Each Squad in Summer League. Perhaps authentic D-League coaches (or generics who are editable in name and appearance)
5. Summer League/D-League Rosters Filled Out with Available Young Free Agents or at Worst, Editable Generics (including name edit capability).
6. Authentic Arena Sounds: Orlando's Summer League features almost no spectators, leading to the sounds of the game taking center stage. Meanwhile, Las Vegas has fans, but it can be at least somewhat sparse. It obviously won't sound like an NBA arena (something 2K covered well in their first Summer League/D-League go-around).
7. Availability During Association Mode: In addition to MyPlayer, it'd be nice to have the Summer League and D-League available to play and work with during franchise modes, allowing gamers to work out draft picks and potential young signees (or returning youths).
In the case of the D-League, it'd be legit to create an authentic enough environment (rosters, coaches, teams) that it could offer a mid-season respite (at worst) for those who have grown tired of the monotony of playing with the same NBA team each night. MVP Baseball 2005 nailed this aspect. Their minor league clubs were fully editable and featured an enjoyable change of pace.
In its past life, I believe the D-League was largely ignored in 2K by gamers due to its lack of customization. It was real teams featuring very fake players. Being able to fill those teams out authentically makes a world of difference. Allowing gamers to download D-League rosters from 2KShare would be a significant bonus.