Crossovers that make a lot of sense yet are strangely unpopular.

[Crazyassloki]

Come on! They both take place in magical worlds inhabited by cute and peaceful creatures (ignoring how the internet portrays them of course), both worlds are constantly attacked by powerful and terrifying forces some of whom edge into eldritch abomination levels of terrifying who are always defeated by their respective heroes, said heroes although cute are extremely capable fighters and hell star allies isn't the first time kirby has used the power of friendship to defend dreamland, remember the rainbow sword from dreamland 2? That thing was powered by kirby's friendship with his animal buddies and his desire to protect them from dark matter and again it's a sword made of rainbows that's powered by friendship that kind of weapon would fit perfectly in the mlp world!

 

[Paulo.junior.969]

Gravity Falls X DuckTales X Scooby Doo

Most of Gravity Falls is basically a mix of the other two.

 

[ZacharyGrossman273]

Warhammer 40,000 X Stellaris by virtue of being dark science fiction with fantasy elements, though that would probably just end with attempts at genocide from both sides