I remember that when the Project Diva X song DLC came out with Sharing the World extra song, the game producer considered making a new game (based on Diva X) with Miku Expo songs in English or another languages. There was a Famitsu interview about it.
Some surveys were made (both private and public), and with the survey results, they decided about translating first Future Tone PS4, then making Mega Mix on both Switch and PC (also because the 10 years exclusivity contract with Playstation ended, so they were allowed to bring Project Diva to other platforms).
And about Project Sekai, the game itself is made by Colorful Palette, a game maker company based in Shibuya, Tokyo, not owned by SEGA.
The reason that is difficult to see a new game is about the song licenses: royalties have to be paid for each new song included in the song list, and especially for new songs by popular producers, the royalties are so high that is difficult to find a good balance between game development costs and number of copies sold.
Future Tone and Mega Mix are based on the Arcade, that added about 3 songs/month during many years, that's why it has so many different songs in one single package.
For me, one easy way for SEGA could be releasing a sort of collection of previous titles, like collection n.1 (PSP titles), then collection n.2 (Project Mirai titles), collection n.3 (Project Diva F/F 2nd/X), and then a new paid Edit Mode (similar to the one of Diva F 2nd) that will allow fans to make new PVs. By giving a fully featured editor to the fans, there won't be the issue about game devlopment costs and high song licenses.
There is also the plan to release a full version of Project Sekai on PC and consoles, but that depends a lot on the higher-ups at SEGA deciding about it.
(Project Diva series in total sold more than Persona series, and as now, Sega is bringing more Persona titles to the various platforms, so that could happen with Project Diva too)
