I have one Spell Shield ability with 600 levels.
Each level has the cooldown set to 0.1*level.
All spells have an Engineering Upgrade equivalent to swap the activation ability of that spell to the spell shield.
After they are swapped, I set the level of the spell shield to the cooldown *10.
Then I have every cooldown between 0.1 second and 60 seconds.
If I have an ability with a longer cooldown, I just increase the levels of that spell shield ability.
(Loading time isnt that bad as the Spell Shield ability has no changes to the text or whatever, so it loads perfectly fine, except the preload at the start of the map.)
The best way to make spells is triggering them.
That way, you can create the entire behavior instead of sticking with a WC3 ability.
The Item ability that you mentioned does exist.
I think it is called poison cloak.
The ability with AoE damage and a cooldown might exist but I cannot thing of an ability right now.
Triggering that one is pretty easy though.
You just create a channel based ability and on activation, you add the caster to a list of units.
Each x seconds, you loop through that list and damage nearby units.
With JASS, you might want to use timers to have the intervals set up properly.