MS best idea was to create Source Generators. Absurdly powerful for some problems.
They still miss something like good old T4 templates though.
Yes, that is missing. I thought about creating something like that, but it's a lot of work.
Also, the huge number of source generators have a problem: a SG cannot see code generated by a another sg. There is no ability to stack them. That will be a problem as more and more sg's generate source. E.g. I have a SG which creates interfaces. No other sg can use these interfaces.