As far as I've been able to see so far, there are no other versions of elementals or different slimes, slime pits spawn slimes all year round (excepting blood slimes), fire elementals spawn all year round. I propose a non-trivial change to further integrate the importance of element vulnerabilities and resistances.
In Spring, Summer, and Autumn spawn Fire Elementals, which AFAIK already have an interaction with rain, but in Winter, always spawn Ice Elementals.
Conceivably, these are the same as fire elementals, but with magic ice damage, high magic ice and ice resistances, as well as probably slashing, with a vulnerability to crushing or lack of resistance.
Additionally, if possible, I also propose a change that lightning bolts striking outside of the Village Area spawn Lightning Elementals, either singular or in small groups. (If you do groups, I'd suggest the design principle of nomad spawns, cap VERY low per spawn) Or just when in lightning storm ---> All Fire Elemental spawns are Lightning instead, if the other is too much work or what have you. Lightning would be much closer to fire elementals, as I know the element is relatively unintegrated right now. Only changing damage to lightning, and resistances from fire to lightning would be sufficient. (I do think spectres should be worse off against magical lightning if only to give the element some utility)
I haven't touched on slimes because I'm not entirely sure if they don't already change per season, as I know they have a lot of colour variety. I believe a seasonal change would apply to them as well, normal in spring, nonmagical fire&resistances in Summer, lightning (and maybe a higher physical resistance?) resistances in Autumn, follow by nonmagical ice damage, slightly slower movement (giving more time to kill them if you aren't being overwhelmed, reducing the impact of the buffs), ice resistances and a small increase in Crushing resistances in winter.
I should note I don't think ALL slimes should become their seasonal types in each season, I know you may think that is a bit much. The additional variety of slimes though would be interesting.
I don't believe I saw this either, but Skeletons could have a bone-throwing variant, that attacks units, but not walls.
Thanks for your time, I'll post more if I think of additional ways to make the systems kiss!