Minecraft's current system of spawning and despawning monsters works pretty great, except in adventure maps.
Sure, you can place spawn eggs in dispensers or hack in spawners. But that doesn't provide the same flexibility as being able to place enemies precisely where you want them, just the right number of them, and know that they'll stay there as long as necessary.
What we need is obvious: non-despawning versions of hostile mobs.
There should be alternate versions of at least some of the hostile mobs, that would look and act exactly the same, but would be immune to despawning. Even on peaceful mode. This non despawning mobs would never be created naturally by the game, but would only be created through the use of an alternate set of spawn eggs.
This would be a huge boon for level designers. You could put enemies wherever you want, no fear of them vanishing if the player moves the wrong way. And players on peaceful mode would only have to fight the mobs you carefully placed, without having to worry about them coming out of any dark area.
What if there was a flag in the meta data for the mobs that allowed or disallowed despawning? That would mean that a load of new mobs wouldn't have to be created, and the mob eggs could just create mobs with the flag set to disallow despawns. That would save quite a bit of work, I imagine.
Yeah, I imagine this would be a very easy addition and probably could be the same mob with different data.
Actually seems like a good idea.
Some people would just switch to peaceful mode to disable all the mobs and switch back to. Cheat on games but this could prevent them on adventure maps.
For always, Support.
These mobs would also be immune to despawning from peaceful mode.
One big problem in adventure maps is that unless you blanket everything with light mobs can spawn in unintended areas. So a lot of adventure maps are intended to be played on peaceful mode, just to avoid creepers blasting holes in important areas, or zombies attacking while you're solving a puzzle.
With these non-despawning mobs, you could play on peaceful mode and still have combat, but just against enemies the map designer planned for.