It is an interesting idea, however I believe with the current rulesets it will be only few cards in many battles.
F.E. when you can use only Odd (or Even cards) 50% of your collection are already out of the race, when you in addition have a ruleset like Magic Only or Range Only how many cards will stay if you can use only 10% ?
I guess in most situations with this rulesets you won't be able to fill the 6 positions.
Also when neutrals are excluded you will have just a few cards, you have approximately 80 cards of every element, so 10% means arround 8 cards for 6 positions, really not a big choice.
So I think when this suggestion should go live their should be a minimum of available cards, I think that for 6 positions it should be at least 10 cards available, so that you can choise at least a bit and not just decide about the position of the cards as you have only 6 (or even less) cards available.
In the modern format this problem will even increase.