according to literatures, the arch-antagonists are: Surt, Fenris, Jormungand, Hel, Garm, legions of Jötnar and dragurs on Naglfar
Utgard-Loki, Nidhogg as well as Fenris' two sons, Hati and Sköll, are the facilitators without opponents and the whereabouts during and after battle.
where will be the main battlefields in Ragnarök ? the fights are very likely to scatter to more than one place, and should fit to the characters
if it's cross-pantheons involved with Olympus and Duat, who will be the antagonists from other religions ? what do you think the matches will look like ? (on condition that they go against the prophecy, Loki may well seek to change his death in Ragnarök, too)
and don't forget the Fimbulwinter, which's the prerequisite for Ragnarök, so why don't start it at first ?