Something I'd like to see introduced, a method to allow members to choose which groups they are in.

My site (in signature) caters for all Fords.

I'd like to be able to allow members to choose which models they are interested in (eg, Mondeo and Focus), and they then become members of those groups.

Then, I can set the forum permissions so that they only see the relevant content they are interested in, instead of everything which is currently how it is.

All we would need for that is to use IPSubscriptions and have a free option. IPSubscriptions already moves members into groups you choose once they subscribe, you then could create forums only they can see depending on the groups you set up/

That would probably be a good workaround solution (assuming you can be in more than two groups or secondary groups?)

Don't think you can at the moment, but I'm sure that could be worked in as well.