Unfortunately there aren't any whispers about either happening anytime soon, but I for one would really like to see both a new NBA Jam and NBA Street game on the current generation. While I do love the sim-oriented games, and want to see them continue to strive for more and more realism, I think there's always a place for two-on-two and three-on-three arcade-style basketball games. Beyond appealing to more casual sports gamers, I also feel they provide an entertaining change of pace every now and then for hardcore hoops gamers as well.
I feel the biggest challenge in both of them is to avoid the repetitive play that's plagued many of their predecessors. There are some classic Jam and Street games, but a lot of them come down to repeating the same strategy until the final buzzer, or hitting the target score: push the opponent to steal the ball, score, and repeat. To that end, NBA Jam: On Fire Edition changed things up with its Real AI, though there was still room for improvement there. I also think there would have to be a few different modes in addition to the campaign/tournament mode, like the mini-games that the 2010 reboot of NBA Jam featured, as well as the Remix games and their equivalents in OFE.
NBA Street Homecourt had a few RPG-like elements, with stats building and the ability to recruit players. I think that'd be something that would need to be built upon in a new Street game. While I haven't been a fan of the cutscene-heavy story approach in NBA 2K's MyCAREER these past few years, I wouldn't mind Street messing around with that concept.
For Jam, I'd also want to see more dunks. As much as I liked Jam 2010 and OFE, it felt like the players were always doing the same dunks. That's kind of true of the classic Jam games as well, but there were obviously more limitations there. If a new Jam game was to be made, I'd want to see a huge amount of insane dunks, including some stuff we've never seen in Jam before. Hey, maybe bring in Zach LaVine and Aaron Gordon, and get them to do some mo-cap on a seven foot rim!