Vermin

If you want to see a grim Spider-Man/Captain America crossover, Vermin would be the perfect in-between character. This creature that eats people was first an enemy of Captain America, but over the years he has been tormenting Spider-Man all over again. So, there could be a whole movie made around the two superheroes and this hideous villain.

He looks like a human-rat and he has the intelligence of a child. Vermin would make a gloomy appearance in the MCU, and he could be part of an interesting Spider-Man/Captain America crossover.