As an honest question, what's left for Epic after you take out demons or undead?
I can only think of other immortals (angels and devils), fey and aberrations. Otherwise you need to get 'crazy' and mess around with parallel universes and such, don't you?
You have no idea how much I would love Fey and Aberrations. It would be
so much.
Being able to run a campaign from heroic tier to epic, about the Winter Fey (as an example, they are pretty neat antagonists) or actually being able to use Mouseferatu's
excellent Bramble Queen in a campaign? I would weep tears of bloody joy.
Tears. If I want to now, it's simply impossible unless I like making all my own monsters.
Aberrations are curiously underused in epic as well. Aboleths and Illithids only go into lower epic - plus Aboleths are mostly MM1 and suck horribly. Then you have a jump to the star spawn at upper epic and then Allabah. Again, more abberations across levels 20-30 would be perfect for the same reason.
After that you could add more magical beasts, evil immortal/planar organizations and similar. So long as the other creature types there: Fey, Magical Beasts, Dragons (we don't have epic MV designed dragons either) and Aberrants have cover you have what you need. Then you can mix up mechanics with what is already there and well established. A game where I can pick Immortals/Fey/Aberrants works much better than "Well, um... I suppose I could use demons again?".
Also in some ways, newer epic Undead and even Immortals wouldn't be bad. Devils are nowhere near as well represented as Demons at epic. Actually
nothing is. They honestly have been doing such a great job at monster mechanics that almost anything non-demonic would be welcome. Demons are the most expandable and usable antagonists in epic tier, plus are easy to use from levels 20-30. Plus you have about a billion great level 30+ options for demons as well. They can then be backed up by tons of elementals and constructs as a general rule also: So encounter variety with a campaign focusing on demons is
easy.
Anything else? Not so much.