We could do a 2016 theme and see how it does. If sales for that one item are good enough then no further item would be needed.
If sales were not then we could do additional themed items within the 2016 year.
With good advertising [ie everyone here shouting out about it around the net] we could possibly generate enough sales to make it with one item and some other donations.
I do think useful maps would sell better than just 'pretty' maps. Maps people could actually game with. So for them it serves a dual purpose - support a good site/cause and get some good game maps.
That also seems like it would be one of the easiest ways for Robbie to administer, as once the DTRPG account was set up and the product was produced all that would be required is some promoting and collecting the revenue.