Lionel already has one freight wagon they could do something with: the open trucks that are available in the Thomas line. My guess is they are in the same league as their Mk1 coaches, but it's a starting point. Remove the faces, put "GW" on the sides, and offer them in three- or five-packs. The chassis could be used for other wagons, such as those AMCDave mentioned.
I've seen the Lionel Thomas open wagons. And I'd buy a few a cut off the faces and repaint them.....BUT....They look really big.....and while I have not been able to place a Lionel wagon next to a Peco or Lima wagon.....the Lionel wagon is just too large.