I get it. There are 3 variables in indexed shifting: the shifter cable pull, the derailleur mechanical advantage, and the cog spacing. You want the ability to modify the cable pull within the shifters.

I am only aware of 2 takes on this, the Modolo Morphos shifters and Campagnolo Ergopower levers, where there is the ability to change the indexing rings. I have never used the Modolo levers, but have read mixed to negative reviews. Campagnolo has changed their shifter internals several times since the introduction of Ergopower in 1991, so you cannot upgrade certain generations of shifters beyond a certain point. I have spent a lot of time digging around in and servicing Ergopower levers, and I know that you cannot upgrade the old 8-speed levers to take a 11 speed indexing ring. I suppose if you'd drop a few hundred thousand bucks on Valentino, he'd be willing to manufacter a custom indexing ring for say: 12 speeds, but short of that, I think you should focus on easier solutions, such as modifying the length of the cable pull after the cable exits the shifters.

Or thinking completely out of the box, I might suggest jumping off of the n+1 cog merry-go-round of planned obsolescence that the bike manufacturers have hoisted on us for the last few decades. 11 speed drivetrain? It is an absurd joke that has gone well past the point of diminishing returns; it is only meant to sell more stuff.