I like this plugin, but I have some ideas you might want to consider as well:

• Make a cool down for the summon, or make it so it can be set to only be possible to summon one ghast per altar
• Make it possible to summon a mob without an altar, like a spell
• I also agree that being able to define the mob type and amount would be nice

I'm looking forward to seeing where this plugin goes, keep up the good work!

Didnt checked so far whats possible with your Plug, but you want Ideas ?

• Make the Altar Block variable.
• Make the activation variable - lever, pressure plate etc. - redstone cycles.
• Make an effekt on activation, like a few lightning strikes around him, or Fire the Altar for a short period.
• Make the Monster Type variable.
• Make a night/day depending spawncycle. At Night Pig-Zombies, at day a Pig. All variable, of course.
• Make the Amount of Mobspawn variable. For both, Night and Day.
• Add a possibillity to break the Altar with a specified Item.

You would give a lot of RPG Fans a nice option to make some nice Quest content. In use with Npcx, Citizens or any other Quest-System for MC this would be very nice ;)

For example.

I would force my players via a Quest to move to a specific location, at night (to kill a Pigzombie, because at day they will not find/spawn any Zombie there but dozens of Pigs! ), i can hide the activation lever, or make a pressure plate near the Altar to trigger the Pigzombie with a boombastic lightning effekt. My Player kills the P-Zombie, destroys the Altar with his holy book and gets back to the Questgiver as a Hero. What a cool journey! :-P

So, your to do List is filled, happy working :-P

And if you need some more Ideas, dont be shy :)