I believe FC3 will be an upgrade of FC2, bringing all of its planes into the DCS World, probably with a few upgrades to things like flight model. As far as I know FC3 and combined Arms is seperate.
No date for FC3 and CA yet.
The first third party aircraft for DCS World will be the Mig-21. You have to look at the devs post to make sure, but I think he said there will be as close to DCS quality as possible, with a clickable cockpit and accurate flight model.
Nobody has announced working on a helicopter, but one can be made.
I'm not sure about ground vehicles, but it's possible that someone could make non combat aircraft.
I'm not sure about how the new engine will work.
Flying Legends is, as I see it, ED making aircraft from the time period of around WW2.