Hey guys I'm trying to make a Homebrew Haldberd that is like a flame tongue that has Chromatic orb flavour. I would like to find a way for my player to be able to switch the weapons bonus damage type mid-battle once per turn as a free action and also add an AOE to it similar to the Dragonborns' breath attack that can be used once per long (maybe short) rest. I've given creating it my best go ATM and think I have the wording down well but I can't figure out how to put the modifiers onto it in a way that feels good. I've shared my homebrew so look up "Chromatic Halberd" if you'd like to see what I have so far and where I'm heading with it.

I also do have what I think will be some cool scaling with proficiency on it so LMK how well THAT may work and ways I can implement it on DnD beyond

Short Answer:

Everything you want is impossible, but…. (See “Possible Solution” below.)

Long Answer:

• There is no way to add damage to a weapon that will automatically roll with the regular attack, even the official flame tongue doesn’t do that currently.
    •  That has to be rolled separately by the player.
• There is no way to have the system select one of multiple damage types, by adding multiple you are telling the system that (when they finally implement the update) you want it to add all of them all the time all at once.
    •  Those would have to be created as multiple variations of the same item.*
• There is no way to add anything “temporary” to a character sheet. Stuff like having it do some damage some of the time but not all the time, or one damage type sometimes and another damage type at other times is currently impossible.
    •  The player would have to switch versions whenever the wanted to change things.*
• There is no way to add actions to magic items currently. That would include the “free action” switch weapon types, as well as the breath weapon effect you mentioned.
    •  What you could do to imitate that is add a charge and attach a spell (like burning hands) to the item.*
• There is no way to have a magic item pull stats from the wielder. Anything that would rely on a characters Ability Scores or Proficiency bonus (other than a Spell Attack bonus or Spell Save DC) cannot be implemented currently.
    •  If you do attach a spell to the item, and that spell requires a saving throw, if the chariots not a spellcaster the system will only use their PB to calculate the Save DC unless you input a static Save DC. (Notice how many items have a fixed Save DC for their spells? Like wand of fireballs has a fixed DC 15.)*
• There is no way to create magic items that “grow” or get better along with the character, even the Vestiges of Divergence from Wildemount had to be created as separate items to imitate that.
    •  You would have to create different versions of the item for that and the player would have to swap them as they leveled up.

Possible Solution

*The closest you could get would be to create a different version of the halberd, all similar to a flame tongue, but a different one for each elemental damage type. The usual “elemental” damages are Acid, Cold, Fire, Lightning and Poison, chromatic orb includes Thunder too. Include the full description for your “Chromatic Halberd” with each copy so the player can see the whole thing as intended on any copy. Name them separately to correspond to the correct elemental damage type, so one named “Chromatic Halberd (Acid),” “Chromatic Halberd (Cold),” etc.
     Then create a separate version of burning hands for each different damage type, and attach each to the correct halberd with the corresponding elemental damage, set them all with a “Min” & “Max” charges of 1, and give the item 1 charge.     

At that point you’re creating 5 additional, different versions of burning hands (one for each element besides fire), and 6 different versions of the item (one for each element) and the player would have to have all 6 on their character and switch which one they have equipped whenever they change damage types. They could stick them all in a specific container labeled “Chromatic Halberd” and swap them around, but they will likely forget when the charge has been spent or not if they use it and then switch damages and then use it again next session befor the charge resets.
     Alternatively, you could make 1 “master” version of the halberd with the additional damage includes, but not having any specific elemental damage type (but all of them mentioned in the description), and 6 different elemental versions of burning hands added to the one item for the player to choose whenever they use it.

I didn’t look at yours (sorry), so I’m not sure what your “cool scaling with proficiency” is. That doesn’t matter though, whatever it is you would need to then make separate versions to represent that scaling. Since a character’s Proficiency bonus scales from 2-6, you would need 5 versions: a PB 2 version, PB3, PB 4, PB5, and a PB 6 version for the player. The player would have to “trade up” whenever their character’s Proficiency bonus increased. That would mean 5 versions of each of those 6 “Chromatic Halberd (Element)” copies for total of 30 different magic items you would need to create. Or at least 5 versions of the 1 “master” item if you do that instead.

I hope that helps.