I think what will happen is what has always happened which is that the US market will drive developments.
So we will have the existing engines running off unleaded avgas, with FADEC or whatever.
GAMI are apparently dragging their heels on certification but they would be fools to run ahead of the market. Nobody in the USA will run ahead of the market demand.
Here in Europe, despite the "EU consumer protection" firms can bring out barely tested products and have them fail all over the place, and wash their hands of it for the most part (thus protecting the main business) because here you can sue for your economic loss only, which in most private GA scenarios is by definition not a lot and anyway most people don't have the stomach for it. I have just managed to force a conman builder to repair a job worth 4 digits; he is an expert and knew exactly how long to string it out for; only a surveyor's report made the penny drop that a CCJ would be the next step. An aviation company - any aviation company - will be equally adept at this game because most face unhappy customers on a daily basis.
The diesel market is of marginal relevance in the USA because of their very good avgas distribution.
Here in Europe, avtur still makes sense for GA but only for a small number of pilots that do serious touring around the more southern parts; the rest of the avtur/piston case hangs on tax concessions, but only a fool will want to build a long term business model, with substantial capital investment, on a tax break. Exploiting tax breaks is fine for making a fast buck where there is a quick and easy controlled-liability exit strategy if it all falls apart.... hmmmm I may be describing an existing engine/airframe manufacturer a bit too closely there
