Why did Josh Sawyer make a mod for New Vegas?
The difference between a AAA release and indie game. Styg doesn't have to cater to Bethestards.
As for what you'd mod into Underrail, I'd revert the psi nerf.
I think it would be interesting to try a lot of things with psi, the system always felt a bit incomplete and up in the air, and went through many revisions.
For instance there was a time when mindshrooms were a lot more useful, since psi points did not regenerate by themselves, and you could get psi directly from shrooms with the
Shroomhead feat, or by straight up injecting them into you with the old psi booster recipe that only needed mindshrooms and syringes. Basically the new "psi reserves" thingy was already there and in a less annoying form.
There's some remains of this concept still in the game, such as the weird little secret rooms here and there, where you must pass some kind of check, but your only reward is a room with 3 mindshrooms in it.
This shows up in the caves, in that keycard-locked biology room in GMS compound, and some other places. Those shrooms would have been a lot more useful with the old psi system.
It could be cool to explore that with mod tools someday.
Me personally I would have liked to experiment with a system where learning psi abilities is similar to interacting with Ignus in Planescape Torment... there is no Psi Empathy feat that you have to take up front, but instead you must suffer some kind of a debuff with every single psi ability that you learn, max health or resistances or something else.
This opens things up for balancing a fair bit, since cave wizards that learn 50 psi abilities will really feel the impact of doing so, instead of just taking that meh -25% health penalty at start and then just getting endless power growth for the rest of the game.
And you can now specify different penalties for learning different psi abilities, so that the very powerful psi abilities will really kick you in the dick, while the less useful psi abilities have very small penalties.