i would BEEEEEEEEEEG for a limited time kitty kat katarina in the store.
As someone who live outside of the USAm aka Canada, can't attempt to join to those event, and i'm sure there is many players in the world, who would like to go at those event, but can't due to where they living 
As a player who started to play LoL in early 2012, i saw a video of Unavailable skin on youtube, and i just fall in love with kitty katarina X.X
Also, but the least, and i doubt the availability of the upcoming skins on the store, but the RIOT Soccer teams is jsut amazing art @.@ thought, only VERY EARLY players of LoL was able to get it, kinda sad for the 2011 early player and + to not have teh chance due to the lack of knowledge of the game back then.
But, yeah
Also
I know that this idea is probably mostly unnacceptable, but for those who doesn't have any limited edition skin OR legacy skin, it could be cool of RIOT to put like a DISPLAY of those skin, NOT TO BUY, BUT AS A DISPLAY
So people could see those marvelous and *cough* sexy *cough* skin they missed, and when they put the cursor on the skin, they could see when and how they was available
like *cursor on akali RST (RIOT Soccer Team*
Availability: October 2010
Why and How: International Soccer Event
that could be cool idea, but i really doubt that player who own those skin would like this idea