I discuss the idea of stacking the die for advantage in a prior thread for training, it works because there is diminishing returns (the -5, 0, +5, +2, +1, +1) that matches the presumptions of modifier stacks built into the difficulty tables for hard challenges.
I realized the same thing can apply to combat advantage.
basically disadvantage then means you loose an advantage, rather than what 5e is now which is +5/0/-5 slapstick. It accomplishes the same thing, it just takes 4 things to knock off the first -5, before you knock off the last two -5. Much more interesting to wear someone down to make them increasingly start missing, while you build yourself up.
In 4e you have the ability to stack +/- attack modifiers, they just don't call them advantages/disadvantages but that is what they are. Combat advantage/disadvantage is limited to the +/-2 that does not stack. There are a lot of +/-2 combat modifers, but few +/-5 modifiers. +/-5 should be your last resort at the end of the contest, unrealistic that someone is that seriously advantaged/disadvantaged at the start of the fight unless it was a serious mismatch to begin with.
There seems to be a fear of stacking because of the modifier blowouts of the past, but the whole point of using multiple die is it tilts the odds in the same way as stacks of modifiers did, but with diminishing returns and without blowing past the 20 unless you have raw ability. So knowing that you realize that you can stack things.
The problem with the +5, 0, -5 are the odds are completely unbalanced, which only makes sense at the end of the fight. If something is a moderate challenge of 12 if you have advantage you are strong change of success, but if you get disadvantage you are strong chance of failure. It is a ridiculous seesaw back and forth if you look at the odds table, where -() means die min, +() means die max
|AC or DC||-2(d20)||d20||+2(d20)|
If instead you look at it as ranks, that advantage knocks a die out of your hand, then it makes more logical sense for a progression: two die advantage then one die then two die disadvantage.
Basically what the advantage then means is that the moderate challenge (12) becomes the same odds as the easy challenge (8).
Basically what the disadvantage means is that the failure on easy (8) becomes the same as failure on moderate (12).
Now you realize you can progress the advantages, there is no need to progress the disadvantages, because disadvantage simply means knocking out an advantage. It essentially means you are attacking your opponents AC/DC with disadvantage making moderate challenges into easy
If someone has a lot of advantaged then the odds are completed tilted into their favor in fact 5th rank easy is a given, the goal of applying disadvantaged is to swing the contest back into your favor by essentiallying attacking the odds bustin them down a rank. I added the hard challenge so that you can see it is not overpowered the higher tiers are there for quickly diminishing the odds unless you have ability which turns the hard challenge back into a moderate challenge. Stacking gives strong defense against disadvantages for easy and moderate challenges. If you bust all the way down to min of two die, then the hard contest is virtually assured failure rather than a slim chance of success to begin with.
I propose that the initial+5 be like 0th lvl training, when you choose your weapon profiency you have the option of taking that weapon expert feat, the two combined give you the +2(d20) advantage which has improved odds equal to a +5 so the next thing to take gives improved equivalent to the +2 on top of that. Thus the first disadvantage attack is only a -2, the next disadvantage attack is a -5, mirroring 4e progressions such as partial cover is -2, complete cover is -5. If someone invests in the higher tiers it gives them protection against disadvantage attacks as they know only nibble -1 at a time, but of course taking the higher tiers comes at an opportunity cost in giving something else up.