I think Ristar was never given a chance to be fleshed out with a sequel or two. Granted, you could say the same about a lot of Sega's IPs. Nights, Space Channel 5, Jet Set Radio, Samba de Amigo, Billy Hatcher, Streets of Rage, Chu Chu Rocket, PuyoPop to name a few.
What about a more recent example: Rhythm Thief, another charming rhythm game from Sega for the 3DS, with a watered down port for iOS. Shun Nakamura said himself that he has ideas for a Wii U sequel, but of course Sonic spin dashed those ideals straight into the depths of hell, where my hopes and dreams for a sequel went boom.
Shenmue proved that there is still love for at least some of there older IPs, especially anything Dreamcast, whether Sega is involved or not. I just wish some of these characters could escape from the 'Sonic and All-Stars' conga and making brief cameos, Nights and Ristar aren't even bloody playable in all-star racing. it's the same crap with Klonoa.