Experience tells me that forum software is too big for a loose project.
it took about six - nine months to get working/stable forum software last time we tried it.
by which time the people that agreed to be testers had lost interest!
and the results were still very much alpha software.
The forum software was named dot Omega.
I googled it, and found people asking for help using code that I wrote! (very weird, I think the project was alive then, why not just ask me?)
num rows not working? - Third Party PHP Scripts - PHP Freaks
there's a thread with a half working forum display page, completely useless without the files included. I seem to remember that there was attribution headers, which this poster seems to have removed. in fact they are changed from GPL to say that it's commercial.
(in fact I definitely remember that particular page was finished.)
I'd suggest finishing that project but...
dotOmega Forum System - Browse Files at SourceForge.net
there are no files associated with the project. I can't remember where I put the source. most of the original development team is no longer here.
also the market for free forum software is already pretty bloated, there are tons of very mature, very stable forum software packages.
Blog software would be pretty cool. (also I think that the turn around would be much faster, so the chances of ending 2013 with quality software is much greater)
My personal preference would be PHP because it's open source, but I'm pretty sure that learning C# would be a nice challenge. (I like C style languages)
My forte is more in hardware.
I like the sound of a hardware project.
The only trouble is that it may require expertise past coding so is a bit exclusive.
it'd be pretty cool to create an automated what's your weather like thread, where we may design and build very simple weather stations to create a network of citizen weather stations.
Home automation would be pretty cool too. where you may divide the project into people who want to work with hardware to do the actual switching, those who want to write firmware, and those who want to write control software, (that may be run on computers, or android phones/iphones etc). (or even just as a web app)
Hardware projects are also (invariably) going to require that people purchase hardware so that adds a cost barrier to people joining the project.
As much as I would love to do a joint/distributed hardware project. I can't see it happening. Whilst I'm sure that we can all agree that home automation is good. not everyone can actually implement it.
Monitoring also sounds good. what sort of monitoring are we talking about? computer/server monitoring (in software) or environmental monitoring?
A game would be kind of cool. but can we all decide on a genre? when we say puzzle game are we talking sliding puzzle solitaire? minesweeper?
So far for me the winners for good project ideas are:
blog software
or some kind of hardware project.