Why not do it like in Master of Magic so that it cost, say, 25 mana to teleport one item to another hero?

The idea is too exploitive and they might feel it is not worth the effort to produce such a feature. Of course with Call of the Titans you have a teleport artifact, by warping your entire champion army next to your other champion army.

That means they have the ability to transfer a lot of items for a low, low price of 100 mana (50 mana there, 50 mana back). Hence, don't trade items until you have 4 and then the cost is down to 25 mana an item.

I'd rather see something like a caravan/messenger unit. This unit can hold items, just not equip them. Give it a high movement rate, and it can go between heroes, and you still aren't burning a hero unit doing the transport. This would also put a little danger in the trip, making sure you watch where that unit steps, so as to not lose all those items.

Yes.  A messenger unit would be great.  Maybe a modder can see if this is possible.

The only units that have an inventory that I find are champions. Champions don't die and get injuries, but retain all equipment. Champions can also go on quests, shop and do a lot of other things. These mechanics cannot change, even my henchmen die mod isn't perfect.

The way I see what people are wanting is a hero on a horse that can travel between other heroes. I don't see why we need another caravan unit type, but I could create an "invulnerable" caravan that can travel quickly between units. Not that difficult to mod into the game at the moment. But the champion tag will allow this unit to go on quests, collect goodie huts, and everything in between.

The spell above is the quickest way I know of transferring goods between troops. 100 mana may be a bit pricey, but it gets the job done.

In Master of Magic, you had a vault where your Wizard's Tower was (your first city, although I think you could move it later).  The champions could then go back to the starting city and get stuff.  Also, when you got a new thing you had the option of either equipping the item or teleporting it to said vault.

I eventually tend to have too many heroes,  so I usually have a mounted one ferry around items.  It's annoying, but sometimes I want that big hammer I just found to upgrade a guy who is in the middle of punching faces in a wildland or something.