One example I've seen would be Earth Romancer, this platform fightan game that's since gone to (((Fig))) for some reason.
It didn't go viral overnight or anything like that, but I had friends link me the debut trailer and a fair few views and subscribers came out of that.
As I understand it, it just turned up in Reddit's Smash communities and did the rounds from there. I'm not sure if it was posted by the dev anonymously or if it just happened organically, though.The video was well-timed, capitalising on Icons' failed reveal. It probably would've done much better for itself if it didn't have its furshit characters front and center, which is more an issue with public image than actual strategy.
Either way, latching onto an existing community seems like the easiest way to build interest. But that has its own pitfalls and invites a lot of direct comparisons. Especially with Smash autists, christ, they're insufferable.