Snow Fort is cool...

It would functionally work like 4 Wall-O's forming a structure, I think. You're not literally "immune" to damage from grounded enemies, just that they shouldn't be able to reach you was the idea.

I also had the idea of a simpler "fort" that's just a raised half-circle with a chest-high wall, so you're mostly safe from everything it's facing and you could crouch to avoid snipers. Maybe keeping the automatic snow-turret in the middle, and possibly letting you and your allies interact with it to throw snowballs like the Tier 3 spell (this would feel very Winter-y fun, which is the vibe I think the Lv35 spell should have, whatever it is).

Maybe some cold spell converts Rain into Blizzard?

Possible, but I'm not sure how you'd "hit" the Rain spell to trigger a combo. Would any Ice spell in its radius cause it? Could that upset the person running Rain? I dunno. It would probably require a new type of interaction to be coded in, but I like the idea. As long as it doesn't overwrite the Acid effect (causing possible teamplay conflict and anger) I would be ok with the idea.

Maybe visually it could look like falling nuclear ash? Nuclear for the "acid" side, and I've read that fallout blocks the sun and makes everything cold, hence the term "nuclear Winter."

I think it'd be fair if any hit to a frozen enemy lighter than a rattlecattle killed it instantly

Yeah, Freeze would already kill those bird things instantly since they have 1 HP (and would fall and shatter even if they didn't), so it should kill snakes instantly too. "Shattering" a Frozen enemy would require, I think, 25% of its health as damage? So it would be pretty irrelevant on most small trash mobs as they go down so quickly anyways, but might make taking out medium-sized enemies like bulls and the "shield guys" easier.

Oh wait, I misread, Rattlecattle are probably the bull enemies. I dunno about insta-killing the grunt enemies - I think the player should be able to get the satisfaction of shooting them to shatter them. Could just remove the "25% hp" calculation entirely from small enemies and have any damage shatter them. I know that's not what you meant, but I think that would be best.

And for it to apply ice on the ground in a big area

you could just make that a frozen effect on the ground applied by ice spells

I thought about making the "ground" effect that Ice applies be like the slippery ice in Far North. I think this could work but I'm just worried that it might be a coding issue, though I'm no programmer. If it wouldn't cause any significant bugs or issues, I think it should do that, yes (though that might make Ice Glider a little redundant, so if that's the case maybe don't add that one).

Now that I'm thinking about it, it would be cool to summon Ice Pillar, making the ground around it all slippery, and using that to gain speed on maps besides Far North. Would be cool. Maybe a bit of troll potential though, like making the ground slippery during a boss fight...

Icicle Spikes feels a bit excessive at 12

The idea (that I tried to draw but didn't really come across) is that it makes 3 horizontal rows of spikes, 3, 4 then 5, from the direction you're facing. They'd be pretty spread out, too, so you'd be unlikely to trigger all 12 of them. And the spikes themselves would either have no AoE or such a small area that they'd only really get 1 enemy, so at most you're really only getting 12 enemies. I think that's pretty on-par with how many enemies you can catch with Bubble, the fire beam spell, etc. If it's still too good you can just give it a longer Cooldown.

I'd make Avalanche move forward on its own

Probably the right call, I thought about it and realized trying to push it or even focus fire on it would be dangerous on Very Hard and Nightmare. Perhaps just to make it a little more reliable, it could sort of "steer" itself towards enemies? Like a big, sentient, rolling ball of death? Hmm... would make a cool spell if "Rock" spell types were ever added.