We pick skins in several different ways.
We go into an ideation saying, "We want a new skin for x champion" and we discuss potential thematics for that champion - Mecha, Arcade, or even new thematics we've never done. We discuss the hooks of these thematics and then we vote on which ones we think are strongest. Those are then further discussed to come up with what we think works the best. The other ideas are stored away and can be pulled out for future ideations!
We go in saying, "Hey, it's almost time for Snowdown, let's talk about the thematic for this year, and who we think would be good champions to support it." Similar as before, but this time, we toss out champions and the hooks that would make the skin really cool and fit in with the event. Again, we vote and further discuss and other ideas not used are saved for later.
We go in saying, "Here's a thematic we want to pursue - Arcade." What champions work well here, and what's the hook that supports how awesome this idea. Same as before - we vote, discuss, and save all ideas for later.
We also take a look at fan creations in these meetings - both from a folder we keep, as well as doing a quick search on Google. They're also considered when we work on ideating on new content!
That said, we do bring up champions who haven't had skins in a while, just to make sure they're on the radar, however, they may not be the right fit for a thematic we want to move forward with/event we're running with.