I think a support that could put ward- like health packs down would be cool, as in teemo but no damage and Healing over time for ally (only) who walks over it

Orbital cannon. A lot like Leona's ult, but much more damage and global range like Gangplank's ult (and about the same diameter). Applies a 3 second slow to anyone caught in it, on top of the magic damage.

The tradeoff is there's more warning. Maybe 2 seconds. Good for teamfights and zoning the enemy away from turrets you're trying to take.

A resurrection ultimate. Yes, there is Zilean, but I was thinking instead of a guardian angel style ressurection, what if you could use it on an already slain enemy and automatically end their death timer?
They'd spawn back at the pool.

I like the idea of the starcraft mothership's vortex. I like the idea that you can put down an AOE hole that lasts N seconds and anyone that steps into it falls into the void. I think it would be a pretty funny ability.

Can take this ability a step further and even make a wormhole. The ult caster creates an entrance and an exit and it allows you to hop through it to a different section of the screen. It could obviously do something additional, but I think it sounds cool

I had an idea for a wasp champion related to skarner. One of my ideas for an ult was to like latch onto an enemy champ (stinging his brain or soemthing idk) and mind control them for a few seconds. The tradeoff would be that the enemies would be able to "kill" their AD carry (or any other afflicted teammate) while the skarner wasp was on him which would just kill the skarner wasp and not the AD.

Locust Swarm. The sky blackens with a ravenous swarm of Locusts, the Bug Lord flies through his swarm much quicker, and heals off the damage they inflict. Vision steadily decreases as the swarm's numbers increase, before slowly letting back up again. The damage would be large but over a very long period of time.

I say an ultimate that is a global ignite cast on the entire enemy team for double the damage of a regular ignite. Would be lawlz.

An ability which causes all nearby neutral creatures and minions to come to the location of the ult and fight. They attack enemies prioritizing champions. They don't attack ally's or each other. Also creatures or minions within the range of the ult get a buff. All creatures can be controlled at once by using them like a pet. Ult doesn't go on cool down until all the minions die. The ult does not end until all the minions die. Also if there are less than 5 creatures to control in the range buffed minions will appear until there are 5. I think this would be funny and OP. But still...

A wide aoe true damage nuke that scales off of AP.

By wide I mean like the size of a Galio ult.