Now: I know that this mode won't be nearly as competitive as LoL, I know the scenario I will paint is not nearly as likely to happen as in LoL (where boosting&shit are a real plague), but still...
If you can queue up as 3 in a "free for all" mode...what's stopping you from gathering 2 friends that don't care about their ranks (or that you bribed with the promise of a skin or w/e) and telling them to play only to ensure you will do as good as possible?
For starter: that way you can almost NEVER end 8th or 7th (unless you ROYALLY fuck up) and that by itself is a HUGE deal, a MASSIVE one (you are left with 6 possible placements, 4 of them granting lp and only 2 of them costing a small amount of lp)
Then there is the fact that you can play a comp and tell the others two to rack up all the champs of the comp that counter yours, making it way less effective for a third player.
Then there is the fact that you can rig the carousel to a certain extent, not needing to go THAT low to be sure to be the first to pick , etc, etc, etc.
I'll repeat: I really don't think that so many people at the start will go that far in order to boost their chances, but in the middle/long terms that could become a BIG problem.