For me it’s first person puzzle games. I can think of maybe a dozen off the top of my head that came out in the last decade. I especially enjoy when they’re open world. The ability to just quit a puzzle that’s stumped you and go try something else for a little bit is incredibly refreshing.
I’m not sure if I have ever seen a game use the Mega Man X formula. I have definitely seen some follow the Mega Man formula, but I liked the X formula with the health upgrades, armor upgrades, energy tanks, etc. hidden in the levels. I think the market for this formula would be pretty small, because I think most would agree that the Metroidvania formula is a straight upgrade. But I kind of like the smaller, independent levels.