Portals are a good place to have random encounters. In fact, at high levels, portals are the new hallways/roads.
For instance, while putting a door most places is ineffective, putting a door in front of a portal is still an obstacle, especially if it's integrated with the structure of the portal such that wholesale destruction/incorporeal bypassing isn't an option. Now you could just use your own Teleport spell, but not if you don't know where the portal leads.
I've used this idea for a distributed dungeon. The idea was that astral/divination warding an entire large fortress is prohibitively expensive. However, if you have a series of small towers in extremely obscure locations, it becomes possible to ward them with existing spells, or at least make it suicide to teleport in (Greater Anticipate Teleport means arriving to a sword in the back). Connect the locations with portals, and it allows fairly fast movement of your own forces while making any enemies go wade through the entire thing.
Another idea: as players can be much more proactive at high levels, enemies should do likewise. Random encounters don't necessarily mean you stumble into the foe, it could mean that the foe comes to you. Demons are good for this - as long as the party has pissed off anyone powerful (likely), they could have made an abyssal deal to have the party harassed in a non-specific way. And as the demons that get deployed don't need to know why they're supposed to attack, finding out the source is non-trivial.