Blitz2, Swdtech2, Morph2, etc...

I meant map as in, like, map. Magitek Lab, towns, cities, dungeons, etc., like you initially said the restriction was.

If you're repleacing Leap, the relevant code is on C0. Do a search for STA $0011E4, and look for where it's being set as $0F. That's the block of code you need to look at.

Another custom command:

Say i wanna create the custom command "Potions", which is a fraction of the command Item... this one works in a similar way of item, but there will be shown only potions, ethers, elixirs, remedy and similar items.
That means that i don't wanna see unusable items, like a dirk inside potions command.
I don't wanna overwrite item command, but both the commands must have at the same time the items they have in common(so both the windows used from the two commands must show potions).

How could it be implemented?

It looks to me like the code that fills the item menu is at 0x254dc... this code is called before and after battle, and after inventory changes during battle, for example when using Steal. Theoretically you could copy this code, and modify it to skip over any items that aren't in a list stored somewhere else in the ROM. I used a similar approach when I wrote the code to sort Rage names alphabetically... the correct ordering of all 255 rages is stored elsewhere, and that ordering is used to check whether each rage should be added to the menu.

However, this problem is different because you want to keep the old item menu too. And because the item list is quite large, the list is filled only at specific moments like I mentioned before, not every time you open the menu... some in-battle memory is set aside for the item list, so you would need to set aside some additional memory for the new "Potion" list, so that it doesn't conflict with the "Item" list. For this reason, it might be easier to use a command like "MagiTek", because it fills the window every time it is opened, so you wouldn't need to set aside any additional memory.

Either way though, I think that this would be quite an elaborate hack.

It IS way more than elaborate this hack: there are really few chances that i manage to officially declare the starting of project #3, but if that happens... heh.

I'd very like to use something like Magitek, but i need of being able to use that items as consumables... still there's something fishy: if that's so complicate, then how does Throw inventory list manage to be there, without interfering with item inventory list?

It should be a mix between Item and Throw this command, it must be a limited choice, like Throw, but it must be used like an item(using potion, breaking a fire rod...)
In any case the thing i care most is (for example) that even if i manage to cast a fire rod, if i don't want to have ice rod available in the list too, then it mustn't be shown the ice rod in the "potion" list.