If ever I was going to build anything steam I'd go for a traction engine. Reasons being it's something that once done isn't reliant on anybody or tracks.
Steam it when you want, run it, within reason, where you want and subject to the usual rules and boiler inspections you are your own man – or woman as the case may be.
I could get enjoyment out of building a small IC engine which is something that's been in the back of my mind for a long while. However once done and run I would have no further use for it but to me the reward would be in the design and build.
With a loco or traction engine the time scale and costs involved if you are working to a decent scale are so great that unless you are building to sell you are committed to run the thing.
Unless you have a large property to drop a track down you are then reliant on clubs.
A traction engine breaks this reliance other than on the boiler inspectors.
Mind you I have just had a very nasty thought, 
Post WWll the Beeby Brothers at Rempstone Leicestershire had teams of ploughing engines that they used commercially. In an effort to move with the times and save time in the morning raising steam they gutted a couple of engines and fitted a pair of diesels on top of the boiler rescued from a couple of scrapped WWll German Submarines.
**LINK**
Just found this but they don't look like submarine engines to me. Fitted in 1951 they are more likely just truck engines.
These would make good and unique models 