[HoneyBadger]

Just to add more thematic flavour, the T'lann Imass might make a good sacred, immortal summon-not particularly devastating, except in large numbers, and hampered by Undead tag, but with high mobility/flying. This already has precedent, since the first Malazan emperor had some T'lann Imass under his control.

The Fokrul Assail might make another interesting, mid to high level summon, especially if they are an uncontrolled, distance summon. Perhaps for the firstform, appearing as a massive sealed tomb-hard to destroy, immobile-that, when finally destroyed-preferrably by independents or your enemies, when you're not around, transforms into the Assail, an engine of death (100% regeneration, multiple attacks combined with quickness, high strength) who attacks both friend and foe.

The Jaghut, with their time immobility/stasis powers, and stealthiness, might be fun to see in the game, since they could be made unique from all the other "giants" in the game. I don't really see them being a summons though, maybe a multihero?

And remember that the Crippled God was basically a Summons that went a bit wrong-so maybe a very high level blood summons? Unique, ofcourse.