Tropes that are surprisingly uncommon?

I just realized that fusion is a very rare concept in media. The only big franchise that features it prominently or at all (that I can think of) is Dragon Ball. Digimon has some here and there like Omegamon, and I suppose it's common in mecha, if you consider the likes of Voltron as fusions.

I think it's a shame that it's underutilized, it's a nice excuse to come up with pretty wild designs and has some interesting potential for character development and whatnot.