Wither and Harvester should be affected by gravity potions.

[summer urchin]

Literally discouraging player creativity

[peak tapir]

I actually like this, but it would also make these potionsms significantly more valuable if they neatly allow you to sidestep a major portion of these bosses' difficulty, especially if the gravity effect were able to apply to more than just these mobs- Quazar, for example, ties its mobility and a major attack to jumping

But I also don't remember the recipes for gravity, so

Might already be investment enough to be in some form "balanced"

[summer urchin]

Nether bricks

So harvester is only post nether or if you're lucky

For wither you may make it only affected by gravity 2, it has much lower duration

And hitting wither with potions is hard

So you'll probably use gravity 2 arrows

[peak tapir]

Doesn't make sense for Harvester, now that I think about it.

He moves through blocks freely

Stick gravity on him and that doesn't prevent him from flying, that sends him to the Beneath.

[raven lava]

i think you know what the answer to this is

[summer urchin]

No i don't

[raven lava]

take a guess

[lofty depot]

This would be actually really cool in an ideal scenario, where the Wither changes its entire attack pattern to crawling on the ground and becoming more melee-oriented. Alas, such levels of adaptability are too complex for us.
And as Myhty said, without any tradeoff, it would absolutely destroy the balance of these "dodgy" and floaty bosses.

That is not even considering that we have no way to add this in the first place. So that's a no from me. You heart is not in the wrong place though.

[quartz crow]

I can send the wither/harvester to the beneath?

Jokes aside, if you think about it, the harvester with potion of gravity will just make it sink to the void and the wither would do the same but with a creeper exploding every other second slowly heading to the nine hells.

Iirc you can gravity the vexes for them to sink

I tried that strat before but it was difficult to hit them with potions. Very small hitboxes.

[peak tapir]

I think an additional concern with allowing potions, even certain potions, to affect some mobs or the like is that there is a potion diffuser block, or something of a similar name, that applies an inserted potion's effect to every mob in the attached room.

You could potentially create a gas chamber for bosses.

But if this were the case... you'd need to either significantly increase the difficulty to produce these consumables, which would likely mean you'd reach the point where such consumables are redundant at that point due to personal gear progression, or make the application of them significantly harder and requiring further progression, which similarly might render such a methid redundant due to other parallel progression achievements.

I feel like the biggest "concern" with something like this, aside from difficulties with actually implementing it on the software end, is the simple fact that the bosses are largely balanced around gear available as you meet them, and their drop being needed in a very small quantity in order to progress; the bosses do not really drop anything you need in any significant number.

Which basically brings up the primary counterargument...

Adding a method to cheese the bosses either removes the intended difficulty- difficulty that consists of, basically, proving you're capable enough to handle the next stage of the game- or allows for farming and potentially automating [or otherwise vastly increasing the efficient acquisition of] their drops.

But the difficulty is intended to be the way it is, and their deaths are intentionally designed to be special ingredients and a gamestage unlock to permanently unlock forward progression.

So...

Yes, I do personally find the idea of being able to ground the Harvester and the Wither, especially since I've had a significant amount of trouble with those two bosses in particular.

The ability to think outside the box and apply your gear and consumables in such a way to literally disable strengths of the enemy is an awesome strategic concept

However, it's too complicated to add, without enough return or value added, while compromising existing, intentional aspects of the modpack's design

[raven lava]

it is; it’s just that dmc’s idea of “player creativity” is cheesing the shit out of everything lol

[peak tapir]

Water bucket mlg when

[raven lava]

what i would consider player creativity is for example using quicksand or cobwebs as a base defense

[peak tapir]

I need to be able to waterlog my blocks so i can place chests down in a recess, fill it with water, and take 0 fall damage by falling into a half inch of water.

[peak tapir]

Add a hitmodel to the windmills so I can use windmills like a giant boot to smack mobs into the air