Here is an idea I've been toying around with:
Basically it's a Ghostbusters mod, but I don't think you can call it Ghostbusters due to copyright.
Anyways, it should include randomly generated haunted dwellings like haunted houses, mines, or, dungeons.
The mod should include new mobs that are specifically ghosts or ghouls.
You could have a flying mob that when you kill it, it leave slime. I know there are slime mobs out there, but they don't fly.
Now for the weapons
Of course there will be a Proton Pack, that shoots a stream with limited range that damages over time, but only does significant damage to the ghost mobs, and doesn't do damage to spiders, creepers and other living mobs. The Proton Pack cannot actually kill the ghosts, only injure them down to 1 heart. Ghosts with lower health, say less than 20%, move a lot slower than they would normally. The ghosts would slowly regenerate their health if they are not captured or killed by something other than a proton pack. On the other hand, these ghosts could be invulnerable to normal attacks, and cannot actually damage the player either. Maybe the ghosts only do status effects like knockback and slowness. You only go looking for ghosts for the stuff they produce after you trap them and process them.
A Ghost trap that you carry around (like in the movies) you put on the ground, and right click it and it opens up. Any ghost that is moving (slowly most likely) inside the cone limited range of the trap will fall into the trap. The trap can hold 1 ghost at first, but can be upgraded to hold more ghosts.
Once the trap has a ghost, you take it to your containment unit block or structure (like Tinkers Construct Smeltery.) Inside the containment unit, you can process it for various items specific to this mod to upgrade your gear. For example, a trap with a wider cone of effect, or longer range and more damage from the proton pack, or more capacity for the trap, or faster processing/ efficiency for the containment unit. You could also use the containment unit processing to get rare items, like ender dust, pearls or Eye of Ender. I don't think processing ghosts for ore, like diamonds, is in the "spirit" of the mod, so I wouldn't include that.
You could also include more advanced features,
proton packs that can shoot different types of attacks,
A cone area effect attack (high damage/ shorter range), or a rapid fire pulse attack (low damage, longer range)
You could have proton packs require some sort of fuel (like the portal guns requiring ender dust) or you would have to create a recharging station that could infinitely recharge the proton pack.
If you are really ambitious, you could create a world like the Nether or the Twilight, and just call it Ghost World and make it look like those Skymap PvP server maps.
This whole thing could take a long time to describe, but I hopefully gave some good ground work to use.
