replying from the live events section about Project Diva and Project Sekai:
Lol, please be patient...it has been over 5 years. I gave up hope.
It doesn't take that long to make a game like this, they just don't want to (well, because mobile games just sell better). I understand, it is just...a big shame.
mm I think I explained it before, but let's explain it again ^-^
first of all, after Project DIVA X we've had from SEGA another titles of the series, like VR Future Live, Future Tone, Future Tone DX and Mega39s. Regarding Future Tone, it takes time whenever they decide to add a new song or a new module, because they need to test it very well before the release to check that everything is fine. It happened many times that a new module, a new hairstyle or an accessory was blocking the camera view or other glitches, so they needed to fix it manually and check everything again with any PV. And because Future Tone (and also Mega39s) has many, many songs and modules, it really takes time. The team cares about delivering a quality product.
Also, making a single "story PV" or an elaborated dance PV might take from some weeks to some months, because you need to create new assets, you need to make a storyline of the PV, and you need to motion capture (with the help of Marza Animation Planet) and direct all the scenes.
second thing, making a game like Project Diva is really a challenge for the game producer: after the game design has been approved and you got the budget and the work timeline, you have to find the good balance between the available budget, the game development costs and the songs licenses. More a song is popular, and more is expensive to have it featured in the game. The most expensive title was Project Diva f because it was built on a new engine, it had all new songs and all story PV. Thankfully the fans understood its quality and the game sold very well and we got the sequels, and both in F 2nd and Diva X you can see a more balanced budget: Diva F 2nd for example had some songs appeared in the past, and that was good in terms of songs licenses fees (they were already paid for the previous entries).
the same "budget balance" can be found in the concerts like Magical Mirai: during these years of live events, Crypton and SEGA managed to find the perfect balance between location, number of seats, songs selection and length of the concert.
third, in the last 5 years, SEGA went under a big restructuring: the new and young CEO, Haruki Satomi (he really likes Vocaloid, he was there at Magical Mirai 2017, 2018 and 2019 to hear fans feedback) has created a more dynamic team group inside SEGA, where each team can help the others. In fact, Diva team has helped making big games like Sakura Wars, Valkyria Chronicles 4, Valkyria Azure Revolution, Kancolle Arcade. SEGA Japan also moved its headquarters into a new building in Osaki, Tokyo.
the producer of Project Diva Arcade and Mega39s, Makoto Osaki (in the past he worked on titles like Outrun and Virtua Fighter, he's a legend of SEGA AM2 team), also explained that he wishes the next Project Diva to come out naturally, because it is what the series deserves: keep being a top quality game that brings lots of joy to the fans ^-^
they have many ideas about the future titles, some ideas have been in development for years since after Diva X, and some ideas have been put into Project Sekai, and some are still in early stage. They want to offer the best they can ^-^