Marucho will return.
Considering how big of a role he had in the original series, and the fact that he's the only one of the trio to not have a new counterpart, him would make sense. Bonus points if he uses Ventus to compensate for Shun taking his element. Haavik has visited the world of the original series
Given his bizarre nature and his unfamiliarity with the show’s backstory, it wouldn’t be out of the question for him to have dimension-hopping capabilities. Volt, Lync, Mylene and Shadow will appear.
While Hydron does imply the Death Bombs kill people, we see in A Heir to Spare that Mylene and Shadow really were transported to the end of the dimension. Both series already play heavily with parallel universes (especially if the Vestal and Gundalian references to Earth culture and history are any indicator), so it's not out of the question for the dimensions to collide like this.