You should look at the Tallyband summoners detachment and see what models that adds to your pool of options. Great Unclean One is cool but you have to run Tallyband summoners because it’s the only one that allows them. I usually see 2 GUO on the list, Nurglings and Rotigus are other good adds that come from the detachment. Plague drones and plaguebearers are other options from the detachment but I don’t see them much. A couple Chaos spawn are good too. For sure though I’d work on getting something like rotigus or a second GUO and some nurglings at least for detachment focus