I like to believe the Ideal masters is just another name for the 18th Daedric prince or the daedric prince of the undead.
Smart idea, but life and death are the sphere of Arkay, undeath is the sphere of Molag Bal (for the sheer purpose of spiting Arkay). Daedra are also known to be very curious about mortals and love to meddle with them. If the ideal masters were daedra, they'd likely be far
more active in the lives of mortals. They also dont have any daedra as servants. I think the ideal masters are simply another spiritual entity. They're powerful, perhaps more powerful than daedra, but they desire nothing, and understand little beyond their insAtiable desire for souls.
And the giant soul gems are actually the physical link between motals and the ideal masters. Thats what Serana's mother says at least, and why the "attack" those gems use is called "soul drain"