1. Boiler will get you into sheet metal work and silver soldering so another set of skills to learn and equipment to buy. Think how often you will use the boiler once the novelty wears off and its simpler if you want to show somebody an engine running to just switch on the compressor.
2 & 5. I'll group these together as there is not really much between the two except vertical or horizontal, quite a bit will be the same as you have already done on the JC so within your comfort zone while still giving a few new challenges.
3. Both Westbury's engines have been hard to get running for a lot of builders with carb and ignition issues but quite nice engines when they do run and would give you a taste of another form of power.
4. The galloway in 1/3rd or even 1/4 scale is a nice engine, areas that need attention are the press fits on the cylinder/ waterjacket and the ignitor needs a lot of care rather than an easier spark plug engine.
I don't think I could list out all the other suitable subjects but if you want a hit & miss type engine then also worth looking to the US for something reasonably priced and simple such as a Perkins or Hired Man. As for steam Blackgates now have some of the Clarkson castings and the 2×2 vertical or horizontal would be a nice chunky subject as I know you don't want anything too small and intricate which would tend to rule out most things from Bruce engineering unless you scratch build at a larger size but probably best to get another casting set or two under your belt first.
J