I momentarily glance at ads while surfing TV Land and saw that all TT games are based on properties LEGO bought rights to. The latest of those being LotR and Marvel, as well as all of DC in addition to Batman they've owned for a while now.
(Surprising DC v. Marvel hasn't been banded about the TT offices, but licensing probably only goes so far, lol.)
Point is, LEGO shares a LOT of rights. Seeing as how the one-trick pony isn't broke and is horrendously successful as well, the TT games will continue for some time if not indefinitely.
In all honesty, I'd have hoped TT could be given Bionicle one day, but after 8 years... Jesus. :|
One final thing, LEGO TMNT just came out in toy form, so its game form should be wholly expected in some time.