OK, it sounds like you've done at least a modicum of thought on it beyond "wouldn't it be cool if...?".
So here are my thoughts, welcome or otherwise.
First, one of my big beefs with RPGs in general stems from the ur-game, D&D. D&D stemmed from wargaming roots and it's so painfully obvious that it did to anybody who actually played those games back when D&D was being introduced. One of the things that stems from this background is the experience system.
The never-ending staircase of ever-increasing power has a direct ancestor in campaign wargames where units got better as they got more experience. The problem is that even in campaign wargames there were:
- Upper bounds.
- Typically an end to the process. (The campaign ended.)
This didn't happen in D&D games, so you wound up with that ever-increasing levels thing reaching literally godlike levels. (Original D&D had, IIRC, only six levels max … but this was one of the first things dropped as people played the game.)
The current shibboleth that this mimics the development of characters you see in the fictions it was based on is trivially observable bullshit. The stated sources of inspiration (Conan, the Grey Mouser stories, etc.) did not start off their stories with the heroes being incompetent nebbishes who could be defeated by a house cat half the time. The stories started with them being at least heroic and, perhaps, over the course of several novels/stories/whatever they would get a bit better. Most advances in the stories were advances of social construct, not powers and abilities. Conan didn't start off barely able to hold his sword and end off able to fight gods in the stories. Conan started off able to fight minor gods and ended off able to, you know, fight minor gods. But he started as an unregarded barbarian and ended as king.
(Note: I am emphatically not saying that there was no development of character abilities in these stories! I'm saying that the development was far less than a typical RPG character undergoes in the D&D-style RPG vein…which includes Storyteller.)
So my response to your (implied) question of "how do we control endless XP growth" is 无, the Chan (Zen) means of, essentially, unasking a badly-formulated question. The proper question isn't "how do we control XP so that we don't have huge imbalances of power?" it is rather "how do we manage player expectations so they don't seek this never-ending escalator of power?"
Some modern games handle this far better. An example of this would be @Thenomain's and and my go-to example: Fate (Core or Accelerated Edition—FC and FAE respectively in the future). These games have "milestones" that, when met, allow character sheets to change. (I'll use FAE for my examples because it's my favoured flavour of Fate, but FC's systems are pretty much identical, just more verbose.)
In FAE a "minor milestone" lets you choose one of (and only one of):
- Switch the ratings of any two "approaches" [read: skills].
- Rename one "aspect" [no real equivalent; think of it as redescribing facets of your character's social hooks or abilities] that isn’t your high concept.
- Exchange one "stunt" [read: D&D3+ feat, kinda/sorta] for a different stunt.
- Choose a new stunt.
Of these options the only one that increases your character's ability in any measurable way is the last one. And that's a relatively minor increase. The rest are about changing the character's focus, like your example of the guy who picked up a gun skill for a specific reason and then didn't really need it any longer would be renaming an aspect.
Then there are "significant milestones". In these you get to do any one of the minor milestone options plus you may do both of the following:
- If you have a severe "consequence" [long-term injury effect] that’s been around for at least two sessions, you can clear it.
- Raise the bonus of one approach by one.
Note that only one of these involves measurably improving a character. (The other restores a damaged character.)
Finally there are the "major milestones". In these you get to do a minor and significant milestone's options plus any or all of:
- Take an additional point of refresh, which you may immediately use to purchase a stunt if you wish. [Again no real equivalent; this is the rate at which you get back your used-up "fate points".]
- Rename your character’s "high concept" [effectively equivalent to a user-defined character class if you will].
And again only one of these two improves the character (and given the fate point economy of the game, it's a pretty decent improvement). The other merely redefines it.
Under the Fate-style "advancement" mechanism I personally think you have a far better model of character change than the D&D-based models that dominate RPGs. Fate itself is not necessarily a good fit for all genres (and most gamers, given how ludicrously conservative they tend to be!), but I think its advancement mechanism could be kit-bashed into other games in ways that make the power creep built into them less of a problem than they currently are for online play.
