For anyone interested in Project SEKAI, if you haven't checked out the original songs by DECO*27 made specifically for Project SEKAI, you should check them out. I'll put them here:

DECO*27×Shota Horie(kemu) - SEKAI feat. Hatsune Miku (This is basically the main theme for Project SEKAI it seems like)

YouTube video

DECO*27 - needLe feat. Hatsune Miku (This song is made for Leo/need, one of the units in Project SEKAI)

YouTube video

Edit: Oh I forgot about some other songs (not by DECO*27, but still for Project SEKAI)

Giga × Mitchie M - Wah Wah World feat. Hatsune Miku & Kagamine Rin (another main theme song)

YouTube

【MV】悔やむと書いてミライ／まふまふ (Made for 25-ji, Night Chord de.)

YouTube

Neru - potatoになっていく (Becoming Potatoes) feat. Kagamine Rin & Kagamine Len (made for Wonderlands×Showtime)

YouTube

PinocchioP - The World Hasn’t Even Started Yet (also made for Wonderlands×Showtime)

YouTube