I was also contemplating Dragon Force. Feels more niche, but almost anyone with some inclination for rock music has peobably heard through the fire and flames at some point.
Adore Mastodon, but yeah feels more "niche". Couple of years ago I saw them in a tiny, seedy venue - great show but I'd be surprised to see them as a main headliner for example.
Fun to see Papa Roach (and gojira further up) brought up, as they were the two first bands to be announced for next wacken... Aaand a lot of my camp mates went "meh" and "who?"
Thats kinda what kicked this question off for me.