A CPP replacement with those features is a long way off. Working on GHC is currently too unproductive to justify the major investment. Maybe if a research group thinks they could get some papers published that would tip the scales, because PhD person-hours are cheap.
I opened Pre-Pre-HFTP: Decoupling base and GHC - #28 by Ericson2314 because I think that is much lower-hanging fruit. If we could use new base with old GHC, we could make changes to base faster without doing CPP nor running afoul of the 3 release policy, and that side-steps the issue.