I don't know if I'm the only one, but bond of stone seems absent in a lot of my games.
Mainly you should see it (imo) on tanky supports, yet it seems that COTC is just generally better or more well rounded. That may change when the patch hits and COTC is weakened, but even then i still see tanky supports running it over bond.
I think this is mostly due to it being 'invisible power' of sorts as well as not being noticeable in the first place. Every other keystone (bar Warlords which you can actually notice) has some sort of interaction or something you have to do to reap the benefits. Bond is just... there. You don't play around it, the opponent cant play around it, and you never think 'thank god i had bond of stone to save us there'.
In that sense, i think it could use a slight rework like warlords did a while ago. The theme of a defensive key stone that is more about protecting your team rather than you is cool, but doesn't really feel good currently in game. I don't think a flat buff would help because, due to it just being persistant damage reduction, it will either feel non existent or broken. I view it basically as a weaker Knights Vow that is less visible.
As such, making it something different could help, the only idea i can think of is basically a reverse storm raiders where when a nearby ally takes --% of their HP (maybe current HP so it works well at protecting low HP allies) they gain a shield equal to a % of your max HP. This is far from perfect, but it would be easier to notice and to be balanced.
Enemies can play around it in lance by poking, procking the shield and then backing off to re-engage, and it also feels more noticeable. The most inherent flaw is that you have no control over it still and you have to basically hope your allies get damaged so it activates. As such, it isn't the perfect option, but i believe that moving Bond of Stone nearer to this kind of mechanic will work better.
Maybe I'm wrong, maybe I'm right, or maybe I'm in the general ballpark, you cant really ever tell how strong something will be in game whilst it is only on paper. In that sense, there is probably a better alternative than just a shield that is more engaging but I'm just writing off them top of my head. Anyway, that is just my thoughts.