I'm willing to wager that there will be way more than 2 ultimates for REM only monsters. After all, Twinlits only represent 2 of five colors present in PAD.
You're forgetting Trifruit which represents the other colors. Also there's a new Keeper coming soon.
The biggest problem with introducing even more Ultimates on top of the Twinlit/Trifruit ones is Gungho has to make any additional Ultimates "worth the effort" without overpowering the character, but also keeping the Twinlit/Trifruit Ultimates from being irrelevant, otherwise people won't run the dungeon/waste stamina and stones. So at most, you'd get a bunch of alternate color/alternate type Ultimates with similar skills.