The lore mentions that there were Nords who turned away from their native religion, and started to worship dragons.
So it seems entirely likely that it will be in the game
I'm hoping for a dragon worshipping cult a bit like the Sixth House in Morrowind. But I DON'T want them to be some generic evil bad guy type factions that just are there to be killed. They need to have much more subtlety than that - with their own lore, dialogue, quests. You need to be able to influence them, work for them, maybe even join them...
As far as I know most of these dragon worshipers are going to be undead.
There might be some living ones living in the civilization, like the priests of order, Mythic Dawn agents and sleepers...
I can't see there not being such a faction- it's part of Bethesda's "standard TES formula." Just like the 6th house sleepers driven mad by dreams who'd attack you in Morrowind, and the Mythic Dawn agents who'd do the same in Oblivion. I don't think it's so much "will they be in the game" as "what will they be called this time?"
I couldn't have put it any better than a mix of what's in these three posts. Yes, I can imagine such a faction and I hope they'll be interesting enough.