Whenever Mrs. Gaggi sends me down to Kum and Go for a replacement propane tank for the grill, I think of how great it would be if there was a truck to pull up and swap out the tank for me. Same with the two-gallon gas can. It's dangerous to tote one of those around in the trunk of the Jag. Wouldn't it be nice to have a guy drop off a full can and take the old one back?
Obviously there would be some safety and liability issues to contend with, but I think it is a solution to a common problem for most homeowners like myself.
I'm not sure there would be enough turnover in order to make it profitable without charging like $50+ a tank. I grill fairly often and I still only go through 2 maybe 3 propane tanks during a year.
Maybe if you combined it with other consumer "utility" items. I know Culligan will deliver salt bags for water softeners, you could tap into that, plus other routine items like that.