I expect a Diddy Kong Racing Sequel to have a completely new cast of characters outside of the Kong characters (Diddy, Dixie, Tiny, ect). That way they can completely ignore all the rights stuff and add in other Nintendo franchises into the mix, but those franchises being lesser or not as popular ones (Rusty, Nintendo Badge Rabbit, Dillion, ect).
I could also see DK, Candy Kong, Lanky Kong, Cranky Kong and Funky Kong also appear in the game as racers too. It will be the bi-generation kart racing series for Nintendo, with this releasing at launch/December 2016 for the NX and MK9 releasing in 2018 for NX.
I believe the rumors for the title and I expect it to be a big title for the NX this fall. Retro Studios and Monster Games co-developing it for the NX launch, is honestly realistic and lines up with a lot of stuff happening between the studios.
-Monster Games working with the DK series for a while now (Returns 3D port, co-developing Tropical Freeze with Retro)
-Retro having experience with kart racers (designed a new track for MK7, shared new track ideas with the MK team, and made the retro tracks for the game)
-Retro NOT working on a Metroid game, confirmed by Metroid Prime producer at E3 last year
Looking forward to seeing what comes from this and considering Monster Games and Retro are amazing developers, excited to see their cart racer this fall
.