Hard to decide. There are a couple I could see working out quite nicely.
1. Ash Williams
I've finished the third and last season of ASH VS EVIL DEAD a couple of weeks ago... and oh man, that show was absolutely amazing. Gory, funny, great production, cool characters, awesome acting. So seeing Ash in the game with his shotgun, his chain saw and maybe even the Necronomicon... would be pretty awesome.
2. Pennywise
Pennywise is a reality warping Eldritch Abomination in the form of a horror killer clown. That in itself is pure victory. And considering how well the remake did and that there is a second movie in the making, I think they would make Pennywise awesome as well.
When it comes to guest characters I always want to see them being first and foremost visually unique and impressive and more importantly, I want them to have amazing power sets, which allow for a lot of variety (variations). That was my issue with Jason Voorhees and Leatherface, since they're both kinda vanilla and to make the best out of them, you really have to do some major reaching.
3. Spawn
Again, great power set, great visuals... You can have a variation, where he wields heavy guns, one where he has his axe, one which focuses more on his coat and magic... There is a lot you can do with a character like Spawn.
4. Doom Slayer
Badass, who wields like a bajillion different weapons. That guy is basically a living, breathing fatality on two legs. His intro should show a bunch of Tarkatans, vampires and other MK monsters and demons running away from him.
There was a great quote, which describes the Doom Slayer perfectly: "In Dark Souls you're locked in a room with a bunch of demons. In Doom the demons are locked in a room with you."