I guess what I look for in those fusions is when the scale matches, you know? Throwing a world-ending threat from one series into another where the heroes just punch hard doesn't always click. The 'invincible' part usually refers to power level, but a good pairing needs to handle the tonal shift. Something like 'Invincible' crossing with 'The Boys' has obvious parallels, but the satire in 'The Boys' can flatten Mark's more earnest moral dilemmas. A more interesting, though trickier, match I've seen attempted is with 'Worm'—that web serial. Both settings treat superpowers as a systemic, often horrific, force, and seeing Mark Grayson navigate the deeply cynical, rules-lawyered world of parahumans creates a fascinating pressure cooker for his character. The conflict becomes less about if he can win a fight and more about if his kind of heroism can even survive in that environment.
On the lighter side, I stumbled on an 'Invincible' and 'My Hero Academia' fusion that worked surprisingly well. The premise was essentially 'what if All Might was actually an alien Viltrumite?' It used the shared DNA of 'superhero society' but explored how the revelation of Viltrumite history and goals would shatter the aspirational, commercialized hero culture of MHA. The best crossovers for this property aren't just about who can punch the hardest alongside Omni-Man, but which worlds have foundational rules that would genuinely warp or be warped by the introduction of Viltrumites.