Nyarlathotep (The King In Yellow has arrived!)
Hastur (HFS, You thought your nobles were corrupt to start with?)
Personally I do not think any of the Great Old Ones, Outer Gods, or Elder Gods should be found stomping around. The idea one of them just wandering around and finding Dwarf Forts to smash and fighting and killing any of them just sounds ridiculous to me.
I 100% agree with this. The world would be demolished in the blink of an eye by even any of these entities awareness - but the elder gods might actually have to show up and have a picnic for that to happen but, really, no - it does not make sense to put those kind of powerful creatures in the world and keep it true to the mythos.
Cults and non-human species (of which there are way more than necessary) should make up the bulk of the game, with some supernatural creatures (not elders or old ones) as the replacement for legendary/epic demons.
You could always go over the top though, and put in a "BEGIN_HARVEST" raw, which would spawn an Elder or Old one to show up and simply destroy everyone in an orgy of violent destruction, maybe set off magma fills/elevation and flood vallys in fire. That would be enormously brutal -- in other words, perfect for the mythos.
Just my $0.02
I absolutely love the idea, though, keep going! Just...you know....would like to keep it close to the mythos scale if at all possible.