My idea is a very simple one since SK seems only able to produce reskins, this could be made in a way that's right up their alley! First, does anybody know those weird wings of winter and wings of summer that don't really go with any costume? well, to me they appear to be phoenix and ice dragon wings, so I suggest making them fairly fast and with not too much health and if you need to just make it like a beefy griever with more skills like fire/ice breath. This event would happen during the winter and summer solstices, since they're the wings of summer and the wings of winter, makes sense right? So, from defeating the phoenix and Ice dragon mini bosses you would get some "phoenix feathers" and "ice dragon scales", which would be crafting materials for possibly more reskinned gear like a fire themed Divine avenger and an ice themed Gran Faust, And the armor could be cool looking reskins of the already fairly popular armor set, Skolver and Vog. Too go into more detail with the gear, the fire themed avenger could possibly be a "revealed" seal sword ( in which I mean that all that papery stuff is off) with a firey aura such as a Gran Faust but with more of a rid hue to it, and maybe like 3/4ths up the sword the aura could kinda branch out to look like phoenix wings coming off the sword. A similar idea would be used for the Gran Faust sword except with a light blue aura and "spikier" wings. As for the Phoenix helm, I really am just thinking of a bird helmet with flaming feathers similar to the wings of summer, and for the armor it would look as if your arms were wings, and maybe we could have bird feet? Also It would have sword ASI bonus med like it's Vog counterpart. For the ice dragon helm, I'm thinking like an icy Chromalisk helm, and same to the armor too. This would have sword DMG bonus med like it's Skolver counterpart. If you would like to distinguish them more from their counterparts, maybe a hint of MSI bonus in there, but for the rest it would be good. As for the bosses, they would be found in fire/ice themed levels, and would be found in their own little rooms similar to the Punkin King. They would have a number of enemies with them, I would suggest they be weak to pierce, and they are basically stronger versions of grievers since SK can really only do reskins. Welp, I've just been thinking of this idea for a little bit, feel free to change what you want, and hope you like the idea even though it's just reskins.
In the future, try to put paragraph spacing in your suggestions. No matter how well-thought out a suggestion is, no-one likes reading a wall of text.
For all of those at home who skipped to my post, here's a post summary:
Wasn't trying to be rude with this, but I'm good at condensing this sort of thing down to its most basic level.
Now on to the critique!
You say that there is only reskins happening, and then proceed to create an event around something new. I find that rather odd. Reskinning a greaver is one thing, but reskinning a greaver and adding attacks is not a simple reskin, it's a new enemy with a base to work off of. Look at the gremlin incinerators in Compound 42 - they're new enemies but themed off gremlin scorchers.
I will say that rewards should be reskinned gear only. You don't say that the sword reskins deal fire and freeze, but it feels sort of odd that they are themed as such and don't do that. However, a fire DA would outclass normal DA always unless its damage is nerfed, and a freeze GF would be a better version of the triglav. Direct reskin is okay but odd, modified weapon is a rather large no from me.
Similarly, the armors. Direct reskin of skolver = ok. Reskin of skolver + MSI = not okay. If we made it the mercurial demo version of skolver that might be alright (damage low, MSI low), but plopping MSI low without either lowering the damage boost or adding a status weakness/lowering defensive powers is a bad move. Ditto to the vog cob reskin.
For armor appearance, phoenix helmet could be a reskinned winged helm (probably more styled like the equinox winged helm), and armor could be a reskinned brigandine or even a hound coat/flak jacket. Ice dragon could be a reskinned dragon scale set since, you know, that's got dragon in the name already.
I like the basic idea for the event as it makes that promo feel more alive than just... well... UV tickets. As long as the gear stays as reskins or follows other armor conventions it sounds fun to me!