Considering the king-with-snakes from Shahname - he is already in the game as Pretender with exactly the same snakes growing from his shoulders (called Lord of Snakes or somesuch). Shiva, etc. is also already in the game, though these summons are limited to India-based nations... And certainly Balrog-like creatures are plentiful enough (and it's not "thought of as demon" - that's his author's own words)! The (almost the) same goes for 4 Horsemen: they should be Death and it already has Tratarians...
Dragons would be very good, with Western-type ones having stats like those of Pretender dragons & only 1-2 Paths, & Oriental ones having different pictures, slightly worse stats & better magic (including Astral). Possibly Seasonal Power & certainly limit to China/Japan mythology-using nations.
Great Bear I've made as Pretender for my Bogarus-splitting mod. Fortunately the graphics were already here. I didn't make him trampler, but have given him 2 Paw Slap attacks having aoe 1 instead. I think it would be more in style of real bears...
Cerberus probably shouldn't be very large, but should have plenty of attacks, including some magical ones (I'd think 3 dog-head bites, 1 dragon-head which it had instead of tail, plus some Hydra-style as it had snakes instead of fur on his shoulders), plus immunity to undead attacks (various drains - don't know whether it's possible to do without making it undead), Dark Power - it became weaker under daylight, and certainly Patrol bonus!
Another possibilty is to continue the line of trolls - make a spell to specifically summon troll Moose Knights & someone Bogus-like...
Considering monster Pretenders I wouldn't put too much hope in them: their use is based, I think, mainly on Fear/Awe combo, with Awe provided by high Dominion. As actual monsters they would be too weak without such Awe (and there are no misc-slot items to grant such iirc) & extra hp granted by Dominion level...
Considering Sleeper I agree that he should be boosted - probably getting something like Hall of Fame ability(s) for free? After all, he's considered to have much experience...
I also agree that while Niefelheim has no real SC-gaining problem, they (and probably other Norse nations?) should have an ability to summon Rimtursar! And an idea that Greek-based nations should (at least at EA) have an ability to summon legendary Greek heroes was voiced recently... And just a thought: background of Man's Witches and Wardens says that Wardens are magically transformed by Witches to become stronger, etc. Maybe Man's Witches should have an appropriate spell which either summons Wardens or transforms normal infantry (?) into such? Similar transformation should be useful for other nations too - and is more appropriate for Nature magic than summons of unnatural creatures. The only possibility for this I think would be were-creatures: either some Werebear/Wereboar/Weretiger - stronger than existing Werewolf so better thug candidates (possibly not for every nation), or lycantrope which is also a mage of some skill so can self-buff (or both). I would also include some dinosaurs, but they probably should be resricted to C'tis...