Outside the preliminary information of the Prehistoric Rahi and the Element Lords, there was some story potential that could've been fleshed out:
•Velika becoming a main villain
•The Chronicler's Company becoming toa. (Kapura has shown efforts to use his dormant toa powers since 2001. In MNOG he's in the ash forest trying to activate his mask powers.
•The revival of an extinct species
•Some characters had death foreshadowing or hints that weren't finished.
•The actual use of the Vahi.