Full build mode

Thinking of a mode like this:

-Start off with 20,000 gold.

That way, it is enough to buy the full build that you usually will not get to when the game ends.

-cannot leave base until you have a full 6 item build at start of the game

Not entirely mandatory. But to compensate the disadvantages below.

-cannot sell or buy more items when returning to base. Only in death you can do that (kind of like ARAM)

to prevent abuse of selling and rebuying a build to counter your opponent's build. Only for your opponent to sell part of their counter build to counter your counter build.

cannot have more than 1 of the same item

To prevent all AD users rushing 4 IEs. That sort of thing.

Boots are unavailable

Again, not entirely mandatory. But boots aren't slot efficient and fed players usually sell them to my knowledge if they're winning and hyperfed.

Can't think of any other differences for now.

Alternatively, Riot can tweak the item sets so that this mode can detect the item sets for the specific champions the player is using, and start with the 6 items based off that item set. Then no items can be bought or sold for the rest of the game.

If the above is too difficult to program, then maybe something like starting with unlimited gold, buying 6 items, then gold becomes 0 and stays there for the rest of the game upon leaving base