What is Sheogorath but the Sithis-shaped hole of the world? Sheogorath practically IS Lorkhan. Look at Arden-Sul and how he mantled Lorkhan to become Sheogorath. The CoC mantled Sheogorath, which is, in turn, mantling Lorkhan.
If you can mantle anyone, wouldn't the Temple faction Pilgrimage from Morrowind cause everyone to mantle Vivec?
I said something similar to this a while back,indicating a link between oblivion and the isles ( the events are the same to me ). I was thinking ( maybe to much ) about sheo and jygg. Meaning it was similar to akatosh/lorkhan,but it basically got dismissed by most. I think "lady N" said there was a connection,but maybe not as much as i thought. I haven't looked into it since,but whether it's my obsession with the isles or not,sheo,jygg,akatosh,lorkhan is important to me. I also mentioned dagon,but sort of backed away from that,for now. To me however you look at it,sheo and dagon are always in the fray somewhere. What got my attention after my post also,was sheo was or is the void shaped hole in lorkhans chest.Something like that. I'm nowhere near as knowledgable as most here,but sometimes no matter how small an idea or connection is,that could ultimately lead to something bigger or something we never thought about.
Sorry to have babbled on.