Brent Simmons, my former programming partner at UserLand, has been thinking out loud about rebooting Frontier. I've read his comments, let them sink in, and just emailed him a response. It makes sense to keep it all public, so here's a lightly-edited version of the email.
From there, I'd suggest factoring the outliner from Frontier, and polishing it up, really make it shine as a cross-platform thing.
That's where I started with Frontier in 1988. First the database, then the outliner and then I mixed and remixed. Those are the two core components.