What's the Sweet Spot for Resurrecting a Franchise?

Ghostbusters/Indiana Jones/Police Academy

What would it take for a new Police Academy movie to be halfway decent? The franchise didn't exactly end on a high note — oy, Mission to Moscow — or really have many high notes, but does that even matter? Last Crusade couldn't reach forward through time to save Crystal Skull. It's not clear yet if Police Academy is going to go the Fright Night/Conan the Barbarian route of a strict remake with a new cast, or if it's going to be more in the vein of the would-be Ghostbusters revival, with the old band getting together to train a new generation; both options have their costs and benefits. One wonders, though: Is there a perfect formula for when it's appropriate to bring back or to revive a long-dormant franchise? Does it only work with material that has either fallen out of favor or was never that beloved to begin with (Battlestar Galactica)? Or material that fans have grown to expect will get repurposed every few years (Star Trek)? Is there a sweet spot between "gaaah, no one cares anymore" and "we've demanded a sequel for years out of habit but we know deep down it's not a good idea," where some series could actually be primed for a new version? And does Police Academy maybe — maybe — fit into it?