With a lot of shared themes is it possible Negamon will also be a super ultimate or a slide evolution for Nidhoggmon?
Not really. if Negamon is considered a Superultimate he could be used as an evolution for Nidhogg in a future games maybe, but im sure they werent made as evolutions of a same line.
As long as we know Negamon is the name of at least two digimon, or could also be one digimon with many different forms, I think (and others) that its gonna be an entire evo line of negamons. Nidhogg was just one of Negamon "pets".