I wonder if a distinction will be made (in the not so far future) between getting rated on a combustion plane vs. electric? Much like you get rated for high-performance or twin engines today.
Right. None of which are relevant to turbine aircraft anyways, which is the majority of all planes in the sky.
It's relevant sure but the big things you learn in flight training are how to figure out where in the sky you are and should be, lift/drag stick and rudder stuff, and endless amounts of regulations.
The means of propulsion is important sure but it's a small minority of the info you absorb.
You won't find a lot of turbines in general aviation...
Fuel isn't the dominant cost in flight training. (Source: I bought my first airplane during flight training and have been flying for almost 2 decades now. Fuel is more than a rounding error, but tends to run around 1/4 of my all-in costs.)
As far as career progression for a professional pilot, most pilots don't deal with mixture/carb heat etc as jet engine themselves are mostly fully automatic with a start button and a fuel cutoff switch and a thrust lever. And even some of those are coming out too and will become obsolete within just a few years time.
An airplane engine has a far more steady-state load condition. As a result, fixed ignition timing and mechanical fuel injection with manual mixture control is "good enough" and very simple/reliable. Likewise for carbs and carb heat on the O- engines. It works and works very well.
There is a process for modifying certified aircraft. STC (Supplemental Type Certificate) is the primary means for changing engine configuration. It's some amount of work, but isn't prohibitive. (I have STC'd modifications on every airplane I've ever owned. It's not that big of a deal, if your modification has sound engineering basis.)
Converting from a carb to fuel injection is possible, but many of those conversions involve installing a fuel header tank that the airframe didn't originally have. Sometimes this changes the fuel system for the worse (eliminating the "both" tank setting) and/or puts a header tank in a place to spray pressurized fuel into the cabin in the event of a forced landing. I looked into converting my 182 to an IO-550 (from an O-470) and that was unappealing to me. Instead, I upgraded to an O-520 (still carb'd, but larger displacement).
Yes, and for example you can choose your new-from-the-factory Robinson R44 helicopter with either carb ( Raven I ) or injection ( II ). The former has slightly lower fuel consumption and longer engine life, the latter better performance.
Minor nit: high performance is an endorsement, not a rating. (It's a CFI sign-off, not a checkride.)