In the future it is much more likely that small turbines will become a 'blind alley' except where the power requirements are so large that a more efficient prime mover cannot be used.
They are horribly inefficient and the justification for wasting that quantity of fuel is going to be very difficult to make for aircraft of small to medium size. They are also as you note very expensive so highly unlikley to devleop much further.
Diesels do offer the potential for much more efficient engines - but in aircraft simply making a diesel variant of an existing configuration is not going to work.
Try picking up the 'dual mass' flywheel fitted to virtually all modern diesel car engines - makes your eyes water - and they are there to ensure that the engine drives smoothly and does not trash the transmission.
If Deltahawk last the course they may be pointing towards the answer. Wilksch seem quite a way from it - finally re-designing the internals so that they can afford to actually make the engine. Most of the rest have fallen way the wayside.
Once someone gets a workable configuration then I'm sure others will follow.
Quite why TCM have decided to buy in the SMA 'technology' baffles me at the moment...........