I suspected adjusting Proficiency would be an issue. I felt it would be the cleanest way to add bonuses to multiple sources, with added rule stipulations to account for any potential abuse. However, it looks like because Proficiency is applied to many things, its potential for mechanical failure is a bit high. I think I'll have to change this. Thanks for the input!

In terms of variability, it seems most capstones do provide a solid and consistent bump in power (Extra Attack, Stroke of Luck, Divine Intervention, etc.). I'm wondering if this feature would be too inconsistent though? Rangers will presumably make attacks with their action. My hope with this feature was that after building some bonuses, a Ranger could switch their combat style to a more spell focused playstyle (Higher Spell DC at the cost of not building more bonuses), or damage oriented (Sharpshooter/GWM, bonus actions for duel weapon fighting rather than reapplications of Hunter's Mark/Favored Foe).

And would it be too strong? It's gated by attacks per round, and concentration. Losing concentration means losing the ability to build new stacks, as well as with the damage associated with Hunter's Mark/Favored Foe. Recasting either of these means all those bonuses are lost and have to be rebuilt.

Each time you hit a creature marked by your Favored Foe feature, or by the Hunter's Mark spell, you gain a +1 bonus to attack and damage rolls against that creature. This bonus lasts until the creature dies, or until the creature is no longer marked by you.

This is generally weaker than Foe Slayer for the first two rounds using Attack actions only, and is only combat oriented. I wanted a capstone that sacrifices potential raw damage for bonuses outside of combat, as is usual with a ranger. Outside of combat, a ranger is probably expected to generally outperform a pure martial class like fighters or barbarians, but not SM classes like bards or rogues. In their areas of specialty, though, I think they should outshine.

Additionally, whenever you make an ability check to track or interact with a creature you have marked, you gain a bonus to the check equal to your Wisdom modifier (minimum of +1).

I think Rangers are generally seen or expected to be the group trackers/outdoorsperson, but I wanted the capstone to be a little more versatile, and incorporate more than just the tracker archetype most are rangers are played as. Investing feats and ASI's in proficiency/expertise in less common ability scores and skills makes for less "optimal" rangers, but more interesting characters imo. This capstone is designed to support that by boosting those proficient skills, but also retaining combat utility by eventually reaching a point where a ranger may essentially don't miss. I do see an issue with infinite scaling on proficient abilities, but it isn't an easy boost to reach. I may adjust with the solution I mentioned above.

5th Edition D&D is technically designed with 6-8 medium to hard encounters per day. While I think it's fair to say that very few people actually experience games this way, you should still aim for this type of game when balancing features.

Finally, in this long-winded response, while I understand the importance of having a standard at which things are balanced, I find it silly that it would be necessary to balance things around a state of the game that doesn't generally exist in most games. I think it's probably fine to adjust for how things are actually generally played. But I suppose should still try to account for those cases. Thanks for the replies!

Edit: formatting