I've been doing some variety speedrunning, running whatever game interests me at the time. But I've been lacking a main for a while. What is a good speedrun game that has a decent technical play, long length, and people would want to watch [fascinating game/already established fan-base]? Thanks ahead of time, lads.

First thing that comes to mind is Celeste, though "long length" is pretty vague.

Long length = >30 mins

Super Mario 3D all stars just came out, so it has decent popularity. Three games to choose from, but the market for viewers in those games is a bit over-saturated on Twitch. Keep trying out new stuff until you find your main- you'll know it when you find it.

Honestly, just take a look at the Games page, most of the first batch of games fit your requirements. It's hard to go wrong with them.

@oddtom would love you, but no switch @Quivico good point, thanks