Old traditional methods of home construction (straw bale, earth/sod, cob) might peak your interest - most are free to do if you own land (and can get permitted- that is something that doesn't always fly!) since the materials are just lying there.
I'd lean toward an RV vs. tiny house. Get one used, see how you like it, sell it if you don't. Either an RV or a tiny house is going to be difficult to add onto later. Then if you're inclined to build something, build a nice core plan - put all of your utilities, bathroom, kitchen, etc. in one small area and put your easier rooms around that. Then you can build out to expand a kitchen into, say, a kitchen + dining room; add a master onto an existing bedroom area, or a living room into a larger great room. If you make the walls and foundation strong enough for a second floor at the outset, you can expand upwards as well.
Whatever you do, plan it all in advance - that way you should be able to save time, money, and headache by doing some of the work at the start - your foundation and rough-ins might be in place for the future full-sized house, but you just frame the smaller version and keep the pads for large patios until you decide to finish it, for example.