Although I understand where you are coming from with this, it is very unlikely this will ever happen as it is too easily abused.I'm gonna throw my opinion in here too.
While Inno say this isn't going to be, it should be.
Currently my city looks awful, its filled with some Christmas stuff and some summer stuff, and just looks odd.
It'd be great if I could Store these event exclusive items (as opposed to deleting them and having to wait a year to try and win them back) and keep my town season specific. I cant condone storing all won items like rewarded standard buildings, but buildings like the pole house, the winter markets and the oasis for example should be storable from a seasonal point of view.
I don't fancy having 10+ items all beach themed over the winter event. Yet I don't want to rid myself of these amazing buildings either!
I plead to Inno to consider event exclusive buildings to be storable.
Although I understand where you are coming from with this, it is very unlikely this will ever happen as it is too easily abused.
Part of the game is to find that balance within the limited space your city gives you, being able to store seasonal items so that you have room to build things for quests you normally would not have room to do, getting rewards for this as well, it's just not something that is likely ever to be implemented.
You forget the confectionary, that one will also get an improved one.Don't save seasonal stuff... delete/sell it when you don't want it in your city.
You're going to get new and improved seasonal stuff at the next event.
Only two items not to sell, at the moment, is the Graveyard and the Lovetree.