It is one of the several anomalies in London's public transport infrastructure
that Muswell Hill, like Roehampton, has no rail service of any kind. That
abandoned route has now been partly built over (near Muswell Hill Road)
so it is unlikely to be fully reopened.

A more likely candidate for reopening is the route between Highgate and
Finsbury Park which still exists as a popular walkway (aka public footpath)
as far as the ECML. The flyover bridge of course is long gone.

If that route were re-adopted and a bridge re-installed, your long-championed
route via Finsbury Park through Canonbury Tunnel would become feasible.
Obviously the idea would suffer from not-invented-here syndrome as the
so-called experts would immediately poo-poo the proposal.

In the very very long term, London Underground should examine the feasiblity
of a new Underground line starting at Arnos Groves, proceeding via
New Southgate, Muswell Hill, Highgate, Upper Holloway and Camden Road to
Euston and through the centre of London.