For example the D&D 5th edition levelling requirements are:
The actual progression doesn't make 'sense' from a mathematical perspective and you can't determine how much is required from the next one because of how much you needed last time but it doesn't matter because games do what the rules tell them. In your game world, the same can be true, my book simply describes the situation upon the character sheet to the user.
This means the reader comes across a couple of lines like this which explain to them where the MC is at in terms of levelling up currently and whether he has any skill points left to distribute (presuming you're using such things, in mine the progression is flexible and each level gives you 'x' to spend):
Experience - 25 (needs 100 for level 2)
Skill Points - 1 spent / 4 available
Hopefully, that made sense to you? - I extrapolated somewhat with regards to my answer as I wasn't 100% certain of what you wanted.
PS - If you want a more mathematical progression (ie. one where the user will be able to more reliably predict progression) then I'd recommend looking at the D&D level procession used in the 1st/2nd edition versions.
That would be something like
and so on. It's exponential but the curve isn't too steep. Even at level 20 you are looking at 530, at lvl 40 at 2815, and at lvl 60 at 11207
Grinding Task A endlessly is useless because it has finite utility until you get a new “slot” it can gain exp through, as well as the fact that you cannot use the benefit from the Task A slot until you also get a Task B slot filled as well. Which is the basic resource management of not only needing sufficient amounts of resources but also multiple kinds of resources and is a very simple way of creating complications and creating power level barriers in a very easy way. You can see this whenever there’s a system that requires a special Item or Quest or the like to move past a specific threshold like lvl 50. It’s a good way to limit growth without creating a ridiculous grind which just encourages brute force countermeasures.
You could also oscillate. Every second level you need a different form of exp. Or have two or three sources of exp (combat and non-combat for example) wherein you can’t level using one past a certain point without leveling the other types. Ot doesn’t even need to fit a pattern with every level having it’s own unique requirements that are either universal, or could be individual to certain classes or even personal to every individual. A mixture of all these added together into any level could also work.
The is also the same reason a lot of games introduce and end game version of a previous resource that is required for special higher level stuff. They eliminate exponential power/resource creep (well, they try their best) without actually changing how the system works overall. It’s a temporary solution in most cases, but those end game level resources will always be a lot more manageable on a numerical level than previous ones. They’re usually at least 10 times less common even after you’ve figured out how to get them effectively.
Another thing I like to emphasize on higher levels are marginal differences. I stead of endlessly increasing power level you could make small differences have larger impacts. As people increase in skill they also increase in how consistent their abilities are in real life. Like how in martial arts it gets harder for the inexperienced people to actually tell when someone does something amazingly well because because the physical precision is increased so the difference is actually smaller, but no less important. The more trained you are the harder it is to improve even further which means small increases become increasingly valuable and you become increasingly able to notice them.