After all slots are occupied you can buy a second boot to make a pair, like 
with 
. After the combining happens, they would occupy only one one slot, MS Bonuses from both are halved, but both of their passives would work.
More Possible combinations like: 
And then all champions would finally get a chance to wear two boots without trolling involved, :)
I just want boot enchantments back...
I think boots definitely needs some enchantment on late game as it was before, I usually sell boot when fulll build to buy better items
There is a Boot on DotA 2 that gives equivalent to:
5 AD 100 HP 5% Atk Spd 34 Armor 310 Mana 55 Mov Spd
A Aura that gives 4 HP Regen and 12 Armor amplified to 15 HP Regen and 90 Armor on targets below 20% Max Health
Also it has a Active that Restores 250 HP and 160 Mana of All nearby Allies and Cleases the User
All of it for only 5375 Gold!
Also there is another Boot that gives 100 Mov Spd and a Active that is a Teleport with 45s of CD for 4400 Gold.
As you can see in dota you can invest a lot of money on boots to get great bonuses.
I wish they had boots like that on LoL!
or bring back tier 3 boots. sigh... them good ol days
cass cant wear boots. thats the problem.
Can't Touch This
The idea certainly seems interesting but you have to clarify a few things for me.
First off: What do you mean by "after all slots are occupied"? Do you mean after they are all occupied by items in general? Because then everyone could just buy a cheap item like rejuvenation bead 5 times in order to fill out the slots and create a pair early on. Or do you mean after they are all occupied by finished items? Because then you would only be able to create a pair of boots at 40-45 minutes, after you have amassed enough gold to finish all of your items.
Second off: Can you buy the same boot twice in order to make a pair, or do you have to buy two different boots in order to make a pair? Can I run double Ninja Tabi, or do I have to run both one Ninja Tabi and one Mercury's Treads?
Third off: Is the stats provided worth it? 2200 gold for 30 armor, 10% damage reduction from basic attacks and a 35% attack speed bonus is extremely lackluster. Compared to an item like Frost Queen's Claim, which costs the same amount of gold, provides 60 Ability Power, 10% cooldown reduction, +2 gold per 10 seconds, 50% mana regeneration, a unique passive that grants 15 extra damage and gold against champions and structures on a 30 second cooldown with up to 3 charges which can be improved to also give 50% movement speed after you have made enough gold with the item, a unique active that summons 2 ghosts for up to 6 seconds that seek out the 2 nearest enemy champions and provides vision along the way plus also revealing them and slowing them for 40% for 2 to 5 seconds depending on distance traveled on a 90 cooldown, seems extremely underpowered. Unless it is the absolute last item you can buy, the gold spended on the item compared the stats it gives is not worth it. What are you gonna do with a 35% increase in attack speed at 45 minutes?
In conclusion, I doubt it will ever be added to the game. I like the idea however and maybe it can be incorporated into some form of boot enhancement we saw in season 5, that Riot promised to add back at some point in the future.
Opportunity cost is a very important part of boot design given how much power is in each set of boots.
