I think the long-term solution is to drop V8. The V8 developers pretty much do not care about Node at all. Perhaps Chakra can come to some agreement to keep their APIs stable and if so they can become the new default engine.
There may at some point be an effort to build an 'abstract node api interface' and main maintain bindings to various engines...
...but there's no way v8 will be dropped at any point in the foreseeable future.
What would you replace it with?
This? no way. We're years away from that even being plausible.
Is there any real point in trying to write a roadmap for something more than 2 years (at least) in the future?
No one has any idea what will be going on at that point; there isn't even a roadmap for an engine independent node api; and even when one is made (never mind implemented...), it will certainly not include 'drop v8' as part of it.
So we're talking about some nebulous future in which something may or may not happen after another thing which may or may not happen at some point in the future.
It's pointless to speculate about such obscure possibilities.
V8 isn't going away any time soon.
While not silly is existing at the whim of a single commercial vendor's browser decisions?
Quaint definition.