It would mean to revise the whole game concept: bags, crafting bags and bank. I don't think that it will ever happen, even more as like said by others before me, part of it is linked to monetization, and last but not least, linked to the advantages of ESO+.

I don't see it happening the inventory management mini game, is a big part of what pushes people towards subscribing for the craft bag.

1000 & I'd make treasure maps stackable too (I have ESO+)

The potions from login rewards stack up to 1000. I'd like all items to do that if possible, but I'm not sure how simple it would be.

If I remember correctly 255 is the largest number that can be stored as 1 byte of data, so I suspect the limit was set at 200 because that way every stack takes 1 byte, and presumably ZOS thought 200 looked neater and more logical than any amount between 201 and 255. (Guild Wars 2 limits stacks to 250 items and I think that's for the same reason.)

I was going to say that means it would take more memory to store a bigger stack, but actually I'm not sure that's the case. If someone has 1000 identical potions they're currently in 5 stacks, taking 5 bytes to store, but since the actual limit is 255 those 1000 potions could be stored as 4 bytes of data representing a single stack.

The problem might be that it's not dynamic. Maybe all stacks would then need 4 bytes of data, even if there's only actually 3 items currently in that stack.

(If you're thinking worrying about 3 extra bytes is trivial bear in mind there's probably thousands of stackable items in this game, not just crafting materials but consumables, siege, soul gems, those key shards for IC, stolen items, and so on. Across several million accounts it adds up quickly.)

However I'm guessing here. It's equally possible that back in the early days of ESO's development someone set the stack limit to 200 because it seemed logical to them, both from a computer's perspective and a humans (and those rarely line up) and it's just that no one's reconsidered it since then. (Although it could also be that because it was done early on changing it now would involve messing with code that a lot of systems rely on and could break countless other things.)

I'm not sure I see why there's a stacking limit at all and I don't understand why there are some items that still don't stack.

It's not like there's any logic to it any more.

I do sometimes wonder if it's supposed to be a sub-game that nobody talks about: The Inventory Management Online. It's second only to Riding Simulator in Cyrodiil.

Potions have 100 stack cap.... but crafted Potions have 200. Um... weird...

Crafting items have 200. Why 200 ? I would understand 999, but 200 is just um... arbitrary & kinda random...
At 1st I though that maybe some top level crafted items require 200 crafting mats to make, but that is not the case either. Again... weird...

Not to mention some things do not stack at all. Like transmute geodes. I would understand if container with random things inside would not stack, but containers that do not have random stuff inside... you would think that at least those would stack, right ? But... no, they don't.

Same thing with sieges. I can understand that sieges have HP, so if you have used some, those do not stack... but what about "healthy" ones (full hp, brand new sieges) ? One would think that those would stack, right ? But... no they don't...

I wouldn’t care if the stack was increased but overall am fine with it as is. I would like stack size to be more uniform for consumables, as some potion types stack only to 100 and I think many crafted foods only stack to 100 as well.

For me, my inventory management challenge is more the number of different things I keep rather than being limited by stack size.

for stuff that doesnt go into the craft bag, larger stacks would be nice

i think they should also make stacks more consistent, max stack of poisons is larger than max stack of potions for example (i dont know off top of my head but i think there are also things which only stack to 100 instead of 200, but i could be mistaken)

other things that should stack but dont would be like treasure maps (why do these have "unique" aspect and material surveys do not?) and as others mentioned stuff like enchantment glyphs

The poll says stackable items, not just stackable craft items.

I struggle with inventory management, and spend inordinate amounts of my game-time doing it. Mules, house storage and so on and so forth. It's frustrating and takes a good deal of the experience of actually playing, away. Increasing stack sizes would help with some things, so I'm all for it. I have the banker and merchant, and eso+, but any quality of life improvement is always music to my ears
Poisons already stack to 1000 by the way.