I hope they don't do a crossover with another DreamWorks movie, it would be to confusing . But maybe seeing different versions of themselves, like a universe where Shrek and Fiona stayed human or a universe where Donkey never met Shrek

This is probably the way they bring back Farquaad, and maybe the other past villains.

There is speculation it's a crossover with Trolls because both franchises have fictional creatures, pop culture references, musical numbers, and event the same director. I think a crossover with Trolls would be more manageable and less confusing than something with KFP or HTTYD.

I could actually see Shrek crossover with HTTYD since they are both medieval settings

Yeah but that is likely not gonna happen as they are making a live action remake of it and it's also much more scientific and realistic unlike Shrek which has magic and supernatural elements.