I'd say it went down hill starting with the first year's finale. Which rather nicely exemplifies what was fundamentally wrong with it: great build-up, but they had no idea where to go with it.
My understanding was that the season 1 finale was supposed to be the end of that particular story and that they would introduce new Heroes in the following season except that the cast was so popular they decided to bring them back.
Still, even taken in isolation the season one finale was a pretty weak ending to that story. Not sure I buy the excuse that the established characters were more popular since they did try introducing several new characters. Tried and failed since if memory serves, most of them didn't even last a full season.
BTW, Heroes and Lost aren't the only shows to suffer from lack of forward planning. It almost crippled nuBSG post season 2, with several plot thread being dumped or back-tracked when the realized too late that what they were doing wasn't working.
The only really good example of doing in *right* in this genre is Babylon 5. There the creator had a pretty clear yet malleable idea of where the show was headed and how it would get there. The details were open to later extrapolation, improvisation and even a total course correction but it was all very well held together by the framework that was set down at the off-set.
Making it up season to season or even episode to episode is perfectly fine for purely episodic storytelling. If on the other hand you make a serial TV show and emphasis it's serial nature as a major selling point and motivation for the audience to keep coming back, then you'd damn well better have a plan and stick to it, otherwise it just won't work.
The real test in the end is dose the show bare a second or even third viewing? I must have seen every episode of B5 at least four or five times over the years. Probably the same for the likes of DS9, Firefly, Farscape, Stargate etc. But I've never felt like going back and watching Heroes or Lost again. There's just no attraction. Not just because the ending is know, but because I know the process was meaningless. Most of the "mysteries" were made up on the spot, mostly just for the sake of being mysterious. There's no real pay-off because there's nothing to connect the dots.
I guess what I'm driving at is that unless there's a drastic change in the way they approach the show, I can't see any resurrection handling it any better since the show was pretty much a wreck by the end. You may as well un-cancel Earth Final Conflict for all the good it'd do.