I was thinking that, though it certainly wouldn't be lore-friendly, it would be very nice to have some peaceful NPCs inside gates. Since there are only 7 worlds, each could have a different NPC or group of NPCs. This would give an incentive to actually pay attention to Oblivion gates, rather than rush through them. Depending on how one wants to style it, the NPCs could disappear or refuse to talk after being used once by the PC, forcing the PC to close this gate and move on to another one. Or they could be used repeatedly, giving the PC a reason to keep track of gates and keep them open, a trade-off between fame points/sigil stones and the NPC services.
The NPCs could give unique services, possibly selling unique items on each plane. They could give quests, obviously; either against targets in Cyrodiil or, preferably when considering the goal of giving purpose to Oblivion gates, against rivals in other planes of Oblivion.
I think that this would preferable to simply increasing the number of planes, or placing random bosses/creating unique gates. Traditional ruins serve that purpose better than Oblivion planes do; therefore, the only option is to give Oblivion gates a unique and enduring purpose in the game beyond simple farming. This purpose is best created by quests and NPCs, unless someone wants to create an entirely new game mechanic or something to make Oblivion gates interesting.