I also want to add some of my own predictions here (based on general information from previous works from the cast and crew involved). I intend to make a more elaborate post, but here's the condensed version.
1 - Arthur Pendragon (now King Arthur) will return. I think this is likely because his actor, Justin Timberlake, is one of the stars of the Trolls franchise. All three were also directed by Walt Dohrn, who's directing Shrek 5, making another collaboration between the two a no brainer.
2 - While I keep saying that no Disney specific characters will appear, that doesn't mean that there won't be new Disney slams in Shrek 5. I expect an intense thrashing, ESPECIALLY if they bring back Lord Farquaad lol.
3 - If the Ogre Triplets are going to be depicted older, they are going to get new, younger actors in the roles. I don't have ideas for who could voice Farkle and Fergus, but Felicia is easier to predict. My candidates are Sabrina Carpenter (who is mega popular at the moment) and Emma Myers (best known for her role as Enid Sinclair in Tim Burton's Wednesday).
4 - We're going to see the return of some obscure characters from the franchise, similar to the Super Mario Bros. Movie. For Shrek 5, the best contenders are Monsieur Hood, Thelonious, the Headless Horseman, and at least one of the ogres from Shrek Forever After.
5 - It will look like Puss in Boots: The Last Wish and The Wild Robot. Look, it's an art style that isn't for everyone, but you can't deny that both of those movies earned DreamWorks a TON of newfound acclaim, largely for their new art direction for those films. Plus, it's cost effective and generally easy to pull off.