"Before it is too late" rules out a fair few artists for me, but there are a couple who are on the brink of retirement, I think:

Toto - incredible band, and Steve Lukather is an absolute legend. My understanding is that he is one of the most recorded guitarists of all time and everybody has probably heard him play on something, even if they don't realise it. Would love to see any of Steve's work on GT, especially some Toto stuff.

King's X - my favourite band through the 90" s. Ill health has dogged the drummer over the last decade or so resulting in cancelled tours. They had been due to play Scotland last year for the first time in about 20 yesrs, but it was cancelled. I doubt I'll get the chance to see them.