CL (Castle Lightning) is fine. Splash would make the dmg too good.
Blind Gate is fairly useless, yes, but thorn wall doesn't emphasise asymmetrical races.
I'd also think Earth/Air would be the building, but yeah. Blind Gate honestly has a cool concept but in execution, not so well.
Just brainstorming a few ideas:
Blind Gate could be adjusted to be similar to the old miner wall - maybe 3 bars of health, can be placed anywhere, denies scouting?
Earth+Air makes me think of Sandstorm, maybe Quicksand (Pool that causes units that are currently in it to be slowed (Medusa sized, maybe slightly larger)) disappears after a while (or could stay?)
I think Elementals needs a structure available for 'turtling', or maybe just straight up offensive, needs something to replace its failed Tower Spawn 2 anyway.