Monster Champions like Rek'Sai have not been getting much love because Riot claims "they don't sell", with our last two being Yuumi and Lillia, both on the "cutesy" end of the spectrum. Honestly, I find that a bit disappointing.
EDIT: Many people have cited that one of the reasons that monster Champions don't do well is because of skins. Obviously, Rek'Sai can't have a K/DA skin, but someone like Rell could plausibly have a TD skin. While that makes sense, it goes to show that Riot needs to get more creative with their monster Champions.
Having too many humanoid Champions makes things stale, and I say Riot shouldn't be limiting themselves to 1 monster per year. There's plenty that can be done with non-humanoid Champions.
If I need to put forth a couple of good examples from Heroes of the Storm, there's Deathwing and Hogger. Neither is human (though Hogger is an anthro), and both are GREAT additions to the game. Hogger is basically what you get if you add the Tazmanian Devil to the game, while Deathwing is probably the ultimate expression of what a Dragon hero should be like. Powerful, unstoppable, and so damn awesome. Very unlike Shyvana, Aurelion Sol, or the drakes on Summoner's Rift, who are much less epic. There are plenty of other non-humanoid examples in HotS too, such as Mephisto, Probius, Dehaka, etc.
Of course, that's just examples I'd like to see. If you were to want more monster Champions, what direction would you go in?
I'd go with the Deathwing direction, personally. A big, badass dragon that isn't some half-breed or planetoid. Maybe make an undead one, a Dracolich, to make it unique compared to the drakes.
As for another option, how about a huge swarm of Void-like insects bound under a single consciousness? Something akin to D'Vorah from Mortal Kombat or the Hunters from Halo.
As a third option, how about something akin to Azmodan in HotS? His kit is very unique, his design is unique, and though he's technically a demon he fits the non-humanoid category.
