I think there is a good chance Valenwood might be the next setting. Skyrim benefited from the versatility of the landscape, using mountains to make the whole game world seem larger. Valenwood takes that to a whole different level, running and climbing across massive branches, traveling across a sea of green in the treetops, or descending into the crevices and canyons of root-level Valenwood. It also continues with the gritty feel presented in Skyrim, again, taking it to a whole different level. One of the things ESO did somewhat right was representing Valenwood.
Valenwood is arguably one of the more creepy provinces in TES, a land both beautiful and horrifying, a land with a much darker side to it, inverted tropes and all that.
When it comes to story, it also has a lot to offer. The Wild Hunt presents a chaotic and unpredictable foe, even unstoppable if fully unleashed, unless you "fight fire with fire". A time of great chance under Thalmor rule, the "progressive" Bosmer of the Thalmor versus the traditionalist, and as some in Alinor would say "backwards and barbaric" devotees to the Green Pact. Ayleid enclaves deep within Valenwood, either undiscovered or ignored by Valenwood's foreign overlord, could also play a part, perhaps wanting to compete Anumaril's plan to remake the realm of the Ayleids.