With regards to weekly free champion rotation...

I have no idea how viable this is or if you'd have to make a few tweaks to make it work, but I think it'd be awesome, if say, on Christmas week (or Kwanzaa, or Hanukkah, or Festivus, etc) you guys had fun with the free champion rotation, and made it Dragonslayers versus Pentakill.

example:

Dragonslayer Pantheon Dragonslayer Braum Dragonslayer Vayne Dragonslayer Jarvan IV Boneclaw Shyvana (she counts lol) Pentakill Karthus Pentakill Sona Pentakill Yorick Pentakill Olaf Pentakill Mordekaiser

And anyone using the free to play champs that week automatically defaulted to the dragonslayer/pentakill version.

Alternatively, the Commando set is complete at 5 champions... Commando Lux Commando Jarvan IV Commando Galio Commando Garen Commando Xin Zhao

and the Battlecast set is incredibly popular... and even growing. Creator Viktor Battlecast Prime Cho'Gath Battlecast Urgot Battlecast Xerath Battlecast Vel'Koz and the upcoming Battlecast Skarner and Battlecast Kog'Maw and Battlecast Teemo.

The pool party set is cool, but might not fit with the winter theme, unless you spend your winters where it's warmer. Pool Party Renekton Pool Party Lee Sin Pool Party Graves Pool Party Leona Pool Party Ziggs

There are 4 headhunters, plenty of blood themed skins, some arcade, some debonairs and heartseekers, and other stuff.

Or you could be truly outrageous and use skins from the vault...