To put a bit more of a point on limited items not being limited:
In terms of Tower Unite, as a player, I see TU being a game meant to be enjoyed by all, regardless of time constraints, when people can log-in, and do what they like, when they like, and removing the traditional feel of "grinding". Limited items negate that feeling in a sense, and create an exclusives club, as well as require you to suddenly feel obligated to grind. From a business standpoint, I believe that a buyers 15$ tomorrow is as good as our 15$ X months ago, and there are better ways to reward long-term players. Our rewards are getting to help with ideas and shaping the game through feedback. Some may prefer limited items for it to feel more special, but that comes at the cost of everyone being able to enjoy it.
Costumes/skins - Sure, skins are pretty typical for limited time items. This is a typical feature in the gaming world, this is what the public sees directly and would have the least impact for players that missed them.
The emphasis here is on decorative items specifically though, which was the original emphasizing point. In the end, it comes down to those items being for you, and just you, to place in your condos. You might get a visitor here and there, but 98% of the condo time is your own personal enjoyment and for your eyes & close friends perhaps.. I myself love Halloween, all year round. The more and permanent decor access we have, the better it is. This is where the compromise of making them expensive could fit in, so people have to work for them still.
In some games, limited stuff fits, or certain components fit. Decorative items for your own condos don't so much, and are make it feel grindy, like every other event. That, and I have seen how some build, I am sure there are some who would like to obtain 100+ of certain items.
It would be nice to get some sort of event duration date as well, so people can plan where/when to allocate time :3
Not sure if the devs actually read this, or it should be under suggestions rather