Demons could be made a bit more varied. Mythgen will do that, but some smaller, hopefully easier changes could be made sooner, assuming they don't contradict whatever lore they secretly have:
1. It's a bit weird flavor-wise that all demons are basically the same size of physically massive (twice the volume of elephants), so it would be good to make some of them smaller. Each demon species or unique demon could have a random size between say, 170,000 (50% bulkier than the largest possible humans) and their current max size. Possibly have 60% of demons/species randomly constrained to a range of 170,000 to 3,000,000, since it's more flavorful for the majority of enemies to be smaller than a few "big guys".
2. To prevent that making them more easily killed than now, use GENERAL_MATERIAL_FORCE_MULTIPLIER to make them more durable. I'm not sure what would be a good number; maybe [GENERAL_MATERIAL_FORCE_MULTIPLIER:1:4] (so attacks only have 25% as much force), and potentially higher for unique demons. This would have the bonus of making the ones that are still big (or made of metal) even more fun. Perhaps adamantine and divine metals can be exempt from this durability thing.
3. Give some of the intelligent species (or unique individuals) some magic powers, like what intelligent undead can do.
4. Give all demons some kind of special syndrome effect that is inflicted on all non-demons around them (also excluding angels probably); a mental syndrome specifically, associated with their spheres. This may be something like a temporary personality change, or exuding out a specific negative emotion, like despair, fear, grief etc. Unique demons may have a power to inflict far more long-term (even possibly permanent) versions of these syndromes.
Some forgotten beasts could make use of these changes too (Except for the potential to be much smaller; their size seems integral to their identity, unlike demons), just not always (so essentially, demons would be doubling-up on "forgotten beast traits" often).