Pick a system. Even if you invent it, the point if litRPG is the MC abusing the system.
I just posted help for d20 3.5.
HERE
For example, most people have a move of 30. The toad familiar has a base move of 5, no sprinting. However, wearing stilts makes your base speed 20. No, it does not reduce your speed from 30 to 20. The way the text reads your speed BECOMES 20. So, giving a toad familiar four, two-foot tall stilts increases his speed to 20 and allows him to sprint.
This was the first step to making a frog that could run faster than the speed of sound.
Or you could, via a series of feats, give your toad "glory tongue", which when combined with a necklace of attacks with whirlwind would allow you to lick everyone withing 100 feet. Which combined with a certain poisonous toad could inflict poison damage to all those people.
Or how about the animated crossbow upsized to a ballista with flying and then turned into a familar and given reloading and quick firing to basically give you a giant floating siege weapon shooting sharpened trees at your enemy like a goddamn minigun?
Or abuse of spammed cloud of knives spells?
Or how about how a monk can do unsoakable, unavoidable damage to just about anything out in the open just by dimension dooring 200 feet over the target and falling on it, yet taking no damage because you'er a monk.
Or the cestus, brawling bracers, and necklace of natural attacks on a monk which can allow you to stack up to +87 bonuses on the monk's goddamn ELBOW when the max should be 10?
The point of a litRPG is the many create ways you can abuse and loophole the system, NOT how fast you can level up and become a god.
Knowing that you level by attacking livinv things, and the MC remembering the location of a living candle that cannot die, so he can take it home to hack for days on end, gaining only 1 xp a swing, but leveling in the comfort of his own home is interesting.
Being able to level your stealth by walking into a particular glitched wall over and over is interesting.
Slaying dragons is dull.
The MC should be outclassed by wveryone who did the standard level by hacking monsters route, but constantly able to pull odd shit out of his ass by using obscure classes and unknown game mechanics.
For example, wearing stilts slows you down to 20 but this has no effect on dwarves or halflings who already move at 20. But by the rules, you gain the "high ground" advantage.
Or holding a wall shield gives you full concealment, so nobody can see you, but you are holding the wall shield, so it is in your inventory therefore no one can see it.
Then you have to handle what happens when someone abuses rules this bad because that makes no sense.
Or an alternate is, the mc isnt part of the game so while he cant level, he also isn't limited to one action every 6 seconds. He doesn't have to move according to a grid. Maybe he's flat out immune to ALL magic. Including healing.
Hope that helps.
Or a third possibiliry is crossing over game systems.
Here we haved d20 meets harry potter. Its a low level power gaming munchkin in Hogwarts.
You get the idea.