POSSIBLE DATAMINED SPOILERS AHEAD!
Well, according to datamining, beyond that door lies the enterance to a new biome that is still in development, called "Repository of the Architects" (you might recognize the name from an earlier version of the game, when the enterance to the zone was at the end of the graveyard beyond the door that requires "Key of the architects", which now contains the merchandise category blueprint)
This means that in the next update we're gonna see a fourth boss (whose name was also datamined to be "The Guardian" and atleast one new zone. There are also rumors of another zone called "The Caverns" although it is unclear where its enterance might be. Guess we'll have to wait and see.
On a side note, I think I've seen a couple other 4 boss cell doors appear in other biomes aswell, so that could confirm that they're planning to add a 4th boss to the game.