I was not advocating anything illegal. To market such and idea would require proper contractual relationships with Bohemia and whoever owns the rights to Thomas.
But, if those rights were ironed out and it was done properly, a Thomas the Tank Engine mod (with ofp tanks, bombs, and guns cut out of course) could sell for $10 at Toys R Us, Target, Walmart, or where ever. You would have to up the fog, decrease the polygons, and dumb down the interface to make it work on low end machines for little kids and their parents. Make some cute missions, nice "Toot! Toot!" sound effects and stuff.
Every kid I have met knows who Thomas is. Nobody I know has ever heard of OFP. I would venture to guess it would make vastly more money than OFP itself.