One thing that I think has real potential is a portable split system aircon for workshops and garages. The all-in-one aircon units have terrible efficiency but split systems require expensive installation that can cost as much as the unit itself. This compromise gets you the best of both worlds.

I built one a year or two ago out of a crusty old second hand split system, and a scrap metal frame that holds the indoor heat exchanger at about head height, and the outdoor heat exchanger back-to-back with it with a ~500mm gap. You open the roller door, position the frame with the outdoor side outside and the indoor side inside, close the roller door, and plug it in. Voila! Aircon! It was way undersized (2.5kW cooling) for my workshop but it still made a noticeable difference. Gonna build a 7kW cooling version for this summer.

I think there's heaps of people with garage workshops that would benefit from something like this. You could also make one where the frame slips through a vertical gap rather than a horizontal one, so it can be set up through a sliding door, allowing renters to use one in houses where the owner refuses to install it.