I wonder if there are any games where there is no leveling and equipment is the only measure of progress? That could be interesting.
It isn't items only, as there are other ways to gain power, but I'm working on a game with no xp system.
Of all the RPG systems I've looked at, Burning Wheel has my favorite progression scheme.
It's a dice-pool system. You don't gain xp. Instead, you have to make a certain number of die rolls of certain levels of difficulty in order to increase for a given skill or stat-related pool. These requirements are meant to push your character's limits. You have to pull off extraordinary stuff that is impossible with normal rolls. The way you have to pull it off is through role-playing. You're required to keep a short, running list of your character's world-views, short-term goals, and long-term goals. It's part of character creation, you maintain it throughout the game, and their structure is loosely codified in the game rules with actual gameplay consequences. When you act in accordance with these things, you earn bonuses (and there are different kinds, according to what type of thing you did in relation to what) that you get to keep and use later. If you use those bonuses towards more good role-play stuff, for example you got some bonuses from achieving a short-term goal and use them for something that's significant to a long-term goal, then they tend to multiply. As such, you can significantly boost your roles at moments that make for good storytelling, and this is what you have to do in order to meet the requirements for advancing your character. And unlike any other game I've ever seen, advancement isn't guaranteed. You have to do significant things, take big risks, and contribute to dramatic moments in the story if you want your character to achieve greatness. If you want to get to living legend territory, you'll hit points where earning that next small step in character advancement will only be possible at big finale moments of whole story arcs.
Example: Let's say your strength is 4d6. In order to increase to 5d6 or raise your die level to d8s, your check list will be something like:
Succeed in 5 tests that require 4 successes at difficulty 5
Succeed in 3 tests that require 2 successes at difficulty 8
Succeed in 1 test that requires 4 successes at difficulty 8
In order to achieve this, you'll have to make strength tests at moments that really matter in regards to the world-views and goals you've written for your character. Smash through a locked door to free the leader of the resistance movement against the empire: get an extra die to throw into another test. Use that die to help move a boulder to gain access to a cave where some artifact is hidden that's essential to your quest: Get an automatic success on another test of you choosing. Grab a bunch of bonuses like this and save them for the final dramatic moment where you need 4 successes at difficulty 8 in order to hold up the collapsing support structure of a building while your party fights the final battle against the evil emperor inside. Congratulations, your character pushed his strength to a previously unattainable limit, and defined it as a core element of his character, by using it in ways that directly advanced the plot, culminating in an iconic piece of storytelling. It makes sense in every possible way for your character to advance to 5d6 strength. All because the system
actively motivates good role-playing and facilitates plot, where the vast majority of dice systems don't really care if you're using the rules for storytelling or just meaninglessly playing with numbers.