License for an arcade game

[Pimm Hogeling]

Hi there, anyone that thinks (s)he can help please respond.

I've started developing a new arcade game. I've decided that I want to
keep the project as open as possible so everyone can contribute (code,
graphics, sounds, even ideas), so they can make the game better for
all of us. To me, this also means that others shouldn't be able to use
the code in other works that are not open (such as commercial closed
source projects). I have read that since GPL code can only be used in
other GPL projects it will always remain open, so that sounds good to
me. Keep in mind that I am not and do not have a lawyer.

However, I have two (probably simple) questions about licensing the
contents of my (or should I say "our") game.
First, to make this project as compatible and open as possible I was
thinking about using a dual MPL 1.1/GPL 3 license using section 13 of
the Mozilla Public License. Considering my "as compatible and open as
possible" and (maybe more important) "always remain open" statements
above, would you recommend a MPL 1.1/GPL 3 license, or would a single
GPL 3 or maybe even an other license be better in this case? I can
find enough info on the GPL, but the MPL is still a bit unclear to me.
Second, the game is obviously not only code. It will also contain
graphics, sounds and maybe other resources. How should they be
licensed? Let's say I license the code as MPL 1.1/GPL 3, does this
mean the game (package) can only contain material (other than code,
that is) with a certain license? Could a Creative Commons license do
the trick?

Thanks in advance!

P.S. I'm more than willing to write a wiki article about this once I
get this sorted out, to share my experience.