I hope that both the Dremora and Golden Saints (assuming they're in) have a more supernatural, "automaton" look about them as they had in Morrowind. Where they seemed more like guardians or enchanted spiritual manifestations, rather than another race of ugly fleshy Facegen people.
I highly doubt the Dragonfires (or Martin/Avatar of Akatosh) filter out "friendly" and "unfriendly" portals.
Individual mortals can still selectively summon the Daedra of their own accord, as they have always done. But the Oblivion gates were more like rifts in that Daedra could freely enter/exit them,
and we saw some of the corrupting energies from Dagon's deadlands spreading at their focal points: the ground becoming charred, bloodgrass appearing, and the like. This cannot happen anymore, but there will always be Daedra here and there on Tamriel through the efforts of witches and wayward practicioners. Velorien also has a point in that the portals created by Boethiah and Sheogorath are more like invitations: challenging mortals to try and help the princes. No problems there.