Improved/Greater Magic Missile or Missile Storm

Just for fun I had a go; I put 288-Magic Missile.txt into Temple of Elemental Evil\overrides\rules\spells, setting the max targets to -1 certainly made a difference at low levels but I haven't found a value that just lifts missile limit yet.

 

It's possible that the limit is hard coded for magic missile. There are some weird hacks like that to work around certain other things. It doesn't seem like it'd be necessary, though.

Also, while your file location is the right sort of place, I'm not exactly sure what locations override what. I think the data subdirectory is preferred to the packaged dat files. But it might not be preferred to some Temple+ specific files if you're using that. Or, maybe with the Co8 mod it's looking somewhere other than the top data directory; not really sure.