Realistically you don't want a gauge at all, you want a big flashy red light that tells you if something is wrong. Gauges are great if you need accurate numbers but they're also intrusive and distracting, whereas a big flashy red light that only activates when things are bad is a much simpler and more effective solution. It would be relatively simple to rig up a subtle light into the main dash cluster too.
Sounds a good, simple idea, but how would that work in practice? With AFR surely there are too many variables. It would need to monitor the AFR, Engine load/throttle position and revs at a minimum. eg. A light that simply comes on at 13:1 would be no good as when cruising at part throttle it would be on all the time and if you had it any higher, and you was at WOT at 6500rpm, your engine would be toast.
I think it would cost to much to get a system to work with just a light for AFR. But your right about the gauges being a distraction.
I do think they are necessary in an FI or or Nos'd zed. I have recently discovered that I had a faulty injector and was running lean sometimes. Without the AFR gauge I wouldn't have been able to diagnose it. And I'm sure if Wasso had one, he wouldn't have melted his plugs on the Wales run.