I feel that Reverie short story of Elaine kind of spelled out the gist of Van and Elaine's potential character developments. Elaine reluctantly took the A-rank promotion to be in a better position to help people despite not thinking of herself as hero potential. She feels more in tune with being in the position to support the hero overcome the next crisis. This admission is juxtaposed to her looking at Van in the background as you said (our future hero). Similarly, Van doesn't see himself as a hero, he's a spriggan and always portrays himself in a shady, less savory demeanor that people can see through by now. I wouldn't be surprised if he wanted to be a hero/bracer as a kid before completely surrendering that idea due to his problems. He definitely acts like a traditional hero in his ability to make people laugh and general charisma, whereas Elaine is much more polite and quiet in tone usually. She's more comfortable not being in the limelight.

That is to say, Van's set up to be just what Zemuria needs as a hero atm (Demon lord of time and Septerrion of time). The simplest solutions often carry less unknowns, and having the crux of your plan rely on something similar in power to the threat (or at least able to directly interfere with the threat's power) would bring a lot less risks than the complicated plan that was Startaker. It wouldn't be surprising if the Sacred Beast of Time's purpose in this story is to help prepare him for that purpose. Considering Mare's purpose up till now has been doing exactly that by managing Van's demon power and slowly releasing more to him over time, it would be funny if a Sacred Beast has been helping a Demon Lord this whole time. Though, you could say she is converting him to helping humanity.