In the current version of minecraft, a mob spawner will choose the location in one of two ways -- first, it might choose a random location within an 8x8x3 area centered on itself (this is the default), or second, with commands or MCEdit, it will spawn a mob at a specific x,y,z coordinate.
This idea is for three closely-related changes to mob spawners, that can be adjusted using either commands or MCEDIT:
To spawn mobs within a region defined by two x,y,z coordinates identifying two opposing corners.
A double value indicating how much mob spawn attempts are clustered towards the center, with 0.0 being evenly distributed throughout the region, 1.0 being clustered towards the center of the region in the same manner as the current mob spawners, and intermediate values indicating intermediate amounts of clustering.
A flag to indicate whether the coordinates of the region are relative to the spawner, or are absolute world coordinates.
What would this be used for? The most obvious would be to have mob spawners which are hidden (in the floor, or a wall, or even on the bottom of the world), possibly surrounded by obsidian or lava or even bedrock, but which produce their mobs spread through an open space in a cave or other structure.
I wouldn't want *all* mob spawners to be hidden like this, just some of them, but it would be a fun occasional challenge.
Another interesting use might be to create (very very rare) erupting overworld volcanos, by hiding beneath the volcano a few mob spawners which produce falling_block entities which will turn into flowing_lava blocks when they land. There might not be a practical use for a volcano, but it would look awesome!
This idea is inspired by video by minecraft youtuber SethBling. Note that he had to use 25 of each type of spawner to get that effect, since each spawner targets one specific coordinate to spawn in falling lava or primed TNT.
More important to me are giving spawned mobs armor and ustom loot.
IIRC this is currently possible using command blocks using sites like http://minecraft.tools/en/spawn.php.
Not tried it (yet) but it does seem to allow mobs to have armor & there's a loot table option as well
@OP - I'd like to add another possible option... How about a 'MinSpawnRange' / 'MaxSpawnRange'
Currently there is already a 'spawnrange' that causes mobs to spawn xx blocks away from spawner, but the Min/Max means mob will only spawn between those limits.
EG place spawn @ 0,0,0 but have minSpawn=30 / maxSpawn=40 & the mobs will only spawn between 30-40 blocks away anywhere around the spawner (yep even vertically) If the normal spawning conditons apply (eg passive only spawn on grass / hostile on solid blocks)