You don't have to ask anyone permission to do a theme week. That's a very specific theme week though. Mass Effect is my favorite game and even I wouldn't be able to pull off a successful Mass Effect Week. We also just had Pokémon Week last month.
Typically, the most successful theme weeks are very broad. This lets the creativeness of the community take over and take the week in places you never imagined.
You're free to try, but consider what I said above. I don't think it'd be successful, but you never know. Memes I made that I thought would get 10 upvotes max have ended up on the front page.
