So I've been thinking, it happens sometimes. We all sometimes walk out of base without any items without realizing it. So I thought it'd be cool if Riot implemented a system where when the game first starts you can't leave unless you buy your starting items? Just a thought.
but i want my first item to be a 
...
I know it's not often, but some people like to save their initial gold for an earlier bigger item. Some champ's kits allow this without too much penalty for not getting Doran's.
That's the problem with a lot of suggestions. They sound good at first, but all it takes is that small group of people who play things differently that will make Riot leave stuff alone (that should probably be changed).
I would alter your suggestion and maybe have a special audio tone (or even a pop up) that reminds you that you forgot to buy items if you leave the steps. That way you can still make the choice, but at least know in case you did forget.
