Alright, would there be demons? I'd assume so, but it's a very good region so..
Highly unlikely. Demons, devils, celestials, &c. exist, but they are not and have not been the primary supernatural force in the world for a long, long time. Put another way, finding a tiefling in the Vale would be less likely than a troll wizard.
If you want to buff arcane casters a bit more, maybe let them spontaneously cast one spell per day, or one of each level per day, in place of a spell slot, wizards from their spells known and sorcerers from the full spell list.
This is workable. /me bangs gavel.
Hmmm.
Some way to help Warlocks, maybe, would be nice?
I've been trying to figure out a way to do this, here's my initial thought:
Warlocks can now apply up to three invocations to their Eldritch blast, but no more than two of either Essence or Blast Shape. So for example one could use a Brimstone Eldritch Spear-Chain, or a Bewitching Hellrime Eldritch Cone.
Additionally, when a Warlock gains invocations on level-up at 6th/11th/16th levels, change instances of 'or' to 'and'. At 16th level a Warlock would then have 1x Dark, 2x Greater, 3x Lesser, and 4x Least invocations. An alternative model would be spreading these invocation gains out over more levels, which is what I might go for.
A Warlock who takes the Fey Heritage feat automatically gains the other feats in that tree as they become available. Fey Power and Fey Skin are gained at 1st level, Fey Presence at 6th level, and Fey Legacy at 9th level.