What if illaoi tentacles were untargetable ?

Are you serious? yes it would be broken as hell, she could set up her death zone for melees and have it be completely unremovable.

It'd be almost as broken as zyra plants or heimer turrets being untargetable.

"What if we removed counterplay from this champion"

It's almost as if her ult does that already, and is rather broken

They would require a short timer, so she would only have brief bursts of area control. They would also probably need to be placed faster and more often, with better decision making when it comes to where they show up.

The best way to look at changes like this is through the power budget. All champions have a budget, and your goal is to balance it out. For every increase in power in one area there must be an equal drop in another. If you don't correctly balance it you end up either over powered or under powered. For your idea it would be a straight up power boost, so it would be broken if you didn't pair it with a reduction in power elsewhere.