1.1 Update: This should allow the mod to properly stop magicka regen when under any stunted magicka effect, not just the Atronach's effect named "wombburn". Prior to this, any similar effect which did not use the "wombburn" name still incorrectly allowed magicka regen.

** I DO NOT PLAY / SUPPORT PLAYING MORROWIND OUTSIDE OF OPENMW HENCE THIS IS AN OMWADDON FILE TYPE **
(With that said, converting it to a regular addon should be as easy as literally deleting the ".omwaddon" and typing out ".esp" instead
If there are any problems doing it this way, don't ask me. I don't play, care about, nor know the first step to mod-making for non-openmw Morrowind.)

This mod is an altered version of Purist Friendly Magicka Regen found here:
https://www.nexusmods.com/morrowind/mods/45636
Credit goes to Remiros and Greatness7

The idea behind this is to have the magicka regen based on intellect, but on diminishing returns and have it scale exponentially on willpower. 
For the medium mod, at 50 intellect and 50 willpower, this will give you 1 magicka per 4 seconds.
Hopefully this allows a bit of a fun way to make willpower (for the purpose of regening magicka) really strong if you stack enough of it.

There are 3 files choose to your liking. I recommend medium.

Fast:
Intellect / Willpower
30 / 30 : .27 magicka per 1 second (1 magicka per 3.58 seconds)
50 / 50 : 1 magicka per 1 second
200 / 50 : 2 magicka per 1 second
50 / 100 : 4 magicka per 1 second
100 / 100 : 5.66 magicka per 1 second
200 / 200 : 32 magicka per 1 second

Medium:
Intellect / Willpower
30 / 30 : .06 magicka per 1 second (1 magicka per 14.34 seconds)
50 / 50 : .25 magicka per 1 second (1 magicka per 4 seconds)
200 / 50 : .5 magicka per 1 second (1 magicka per 2 seconds)
50 / 100 : 1 magicka per 1 second
100 / 100 : 1.41 magicka per 1 second
200 / 200 : 8 magicka per 1 second

Slow:
Intellect / Willpower
30 / 30 : .03 magicka per 1 second (1 magicka per 28.68 seconds)
50 / 50 : .12 magicka per 1 second (1 magicka per 8 seconds)
200 / 50 : .25 magicka per 1 second (1 magicka per 4 seconds)
50 / 100 : .5 magicka per 1 second (1 magicka per 2 seconds)
100 / 100 : .70 magicka per 1 second (1 magicka per 1.41 seconds)
200 / 200 : 4 magicka per 1 second

Installation:
Put into your data files folder as you would any other mod and activate it. Remember to change the file type to .esp if you're not using Openmw

Again credit goes to Remiros and Greatness7

Nerd stuff:
This mod checks every 1 second (instead of every 3 seconds like the mod this is based on)

Equations:
Fast: (GetSquareRoot, int) * (wp * wp / 17677.6695275)
Medium: (GetSquareRoot, int) * (wp * wp / 70710.67811)
Slow: (GetSquareRoot, int) * (wp * wp / 141421.35623)