They can't at this point; Grinch is the last Universal Dr. Seuss movie for now.
Warner Bros is the next home for Dr Seuss projects:
Warner Animation Group Strikes Multi-Movie Dr. Seuss Deal
I think this is the best outcome though; Grinch is the most important Seuss character, and Universal got out a modern animated version that it will be able to keep in its library and distribute on TV/streaming services into the future (especially during future holiday seasons).
Illumination has to focus on its current projects: Nintendo, SLoP, Minions/DM, Sing. Meledandri is going to be working on the story for Shrek's revamp at DreamWorks as well.
All of that means there's no space for further Seuss adaptations at Illumination.