This Is a problem almost all other characters share, and basically he should have just appeared more and done more (we don't really know what he did, but still).
For example, he could have been in DR when Fuji called for reinforcements and do something (since he Is a sword member, he can do basically what he wants).
It would have been nice to see him between the arcs capturing people and maybe having his name on papers.
Another lost chance was for him doing something really meaningful against the Shicibukai after the abolition. He could have fought someone and have a breakthrough.
Obv, but that would have changed the character, the easiest way would have been to give him an overpowered fruit. A mythical zoan or a strong logia, but even Jozu's fruit would have been simply perfect for his stile. He would have gained unparalleled defence and a huge offensive boost. Image AdvCoA + AdvCoC + Diamond defense and offense: crazy.
And since Koby has a very advanced CoO, he would have been even hard to hit. And when you hit, you find Diamond + advanced haki.
Yeah, maybe I would have gone for pure haki still, but if I had to choose a fruit I would have chosen Jozu's.
* The same can basically be said for Smoker, minus the fruit part.