This is mostly tongue-in-cheek boredom on a Saturday morning. After tens of thousands of shots at the track I've learned some things about what would be the perfect setup. I thought at some point I wanted/needed speed, MP, etc. etc. Nah...
I like everything about the A7 IV and really don't 'need' them to change anything. However, in a perfect world...
As a rule I'm not a big cropping person and would rather compose correctly. But, if there's a car at the line that I know can launch very fast, I tend to zoom out to give myself a little more time and space for a complete panning burst. There isn't any time to correct once it goes, and if I misjudge it can result in a shot that has too much of the surrounding area in the frame. With the A7R III I know I can give them a bit more space and still end up with a nice sized file after a crop. Having said that, I don't think I want to deal with more than 40-45MP, although if push came to shove it wouldn't be out of the question.
The frame rate is just "Gee, I wish". For a while I had myself convinced I needed a much faster camera, like the Z8 or A1. The M4 maxes out at 6 frames per second when shooting uncompressed or lossless compressed RAW. The RM3 gets 8 frames shooting RAW. 8 frames seems to be a nice balance between "I missed the perfect frame" and "Holy crap! I don't want to cull 10,000 frames!" Having shot drag racing and more recently had an experience shooting ARCA racing, I've learned that not much happens in 1/20 of a second that is missed at 1/8, or for that matter 1/6. I would say the same is true for air shows. It just feels like a nice balance. Ironically, if I were to buy some kind of speed-demon camera, it would need to have the ability to shoot slower!
So, before someone asks why I don't just shoot the A7R III instead, it doesn't do as good a job in the AF department. An 8-frame burst on the M4 will result in 8 shots in focus. That drops significantly on the RM3, although right now I'm trying some different settings to see if it can be improved and have made some inroads. The camera is also a bit too small to hold comfortably for a long period, thankfully they improved the ergonomics on the their more recent bodies.