You aren't alone. There was a creature made a while back, or rather a format for creating generically random things from the Far Realm... Here it is/.

Pseudonatural creatures, half-farspawn, and elder eidolons are pretty awesome elements for a campaign using the Far Realms, but I never liked uvvudaums much. First, they're not all that bizarre, and secondly they don't seem powerful enough to be described as lords of the Far Realms. I don't even much like the idea of there being anything more than a who eats (or otherwise destroys) who ecology in the Far Realms. There certainly should not be a uniform class of beings who consistently dominate parts of the Far Realms. Also, creatures from the Far Realm aren't supposed to be terribly interested in the Material Plane, for the most part; if they were, the Material Plane would be in a lot of trouble. For the most part, they ignore the Material Plane and it ignores them.

So the real problem comes when alienists and other wierd cultists start disturbing things from the Far Realms. Alienists, fleshwarpers, elder eidolons, and a variety of half-farspawn/pseudonatural minions work well for opponents here. Tsochar and aboleths sometimes fit in, too, since they're associated with the Elder Evils. There are some good creatures from the Fiend Folio, too; kaorti as an enemy race, plus a few minion-creatures like rukanyr (or whatever they're called) and skybleeders.

If cultists actually manage to summon something powerful from the Far Realms, its first act should always be to consume most of the cult and then start rampaging. Can't reason with them, and they don't discriminate.

The main problem with an entire campaign centering around the Far Realms is that you have to be sparing with its creatures, or else the players get used to them, which should definitely never happen. If possible, try to make sure they don't know what an opponent is or anything about what it can do, which probably means a lot of homebrewed creatures or templates. Most of the enemies will probably be cultists and alienists, with pseudonatural summoned creatures to throw in a bit of wierdness, and then occasionally something wierder appears.

The Sanity variant from Unearthed Arcana is good too, but it should only be used for things from the Far Realms. Ordinary zombies or ordinary spells shouldn't damage sanity.