SMT IV's huge compendium was largely possible due to using artwork and sprites for battles over 3D models. Nocturne, being the first mainline 3D game in the series, had a very small compendium, and had every single demon redesigned by Kazuma Kaneko to be able to transition to 3D better. Atlus has produced a lot of new high quality 3D demon models because of Persona 5, and SEGA upped that by around 30 for the SMT mobile game, all the while retexturing and touching up the P5 models to have more realistic texturing and modelling, which seems to be the direction they might go for SMTV so we'll probably be seeing those models in that. I believe most of the Nocturne demons are present in the mobile game, though some are using alternate designs like Lucifer using his SMT2 angel design due to the Lucifer in Persona 5 being a JoJo Stand. If they ever remake SMTIV though, I hope they have Doi redo every single one of the guest demon designs. While we got some nice designs like Minotaur, most of them were awful, and they were vastly improved when Doi redesigned the returning ones for Apocalypse.