What about a block that, when placed on top of a vanilla mob spawner and fed emeralds, either causes the entity associated with that spawner to spawn without needing a player nearby or produces items from that mob's drop table?
The main intention with this is that, since vanilla mob spawners are (barring various other mods) immovable, this would give players a reason to add more train stations to their train network, to build long-range infrastructure, or at least local infrastructure to both collect from and ship to these locations.

Idea seems fine, but it feels too "magic-blocky" to me. I would propose a different approach to remote, spawner-based mob generators: a cart that simulates players presence (from spawner standpoint) or just forces spawners to activate. That would require player to set up a mechanism to force mobs into carts to be carried away for further processing.
To further increase needed complexity, a limit on how long at a time one cart can affect any given spawner would prevent people from just parking the "taunt cart" near a spawner and gathering mobs.

well, the requirement for emeralds does lend itself to using the Railcraft trading station (the main reason I suggested emeralds instead of meat, which is another option, or some other fuel) and therefore requires that you set up farms for stuff that you then trade to those villagers. That, in and of itself, can get somewhat complicated, given the pickiness and finickiness of villagers. Therefore, I think it's acceptable if the block itself is a little magic-blocky.

Just my two cents/logic.

Mob Extractor Cart, drop an emerald in the payment slot, drive it by a mob spawn, drive away with brand new mob.

Emeralds for zombies? Price seems a bit steep.

What I see is that we can change vanilla spawners' activation ranges and increase its change to spawn mobs with more exp drops, etc. We can also add a minecart form that can be linked to a spawner minecart. #939