Well see, that's the iffy part here. Such a change would only affect NWC. It wouldn't do a thing for the other bunches of programs and hardware out there that have Timpani where it's at.
Then again, most of us probably don't use anything but NWC anyways as far as midi goes, so it might not be that big of a deal.
I understand why it makes more sense elsewhere, but the General Midi specifications have been set in stone for 23 years now. Changing it may make it easier for newcomers, but would confuse everyone else.
This instrument tree contains all the voices and SFX patches from the standard XG set.
Please note that the official XG groupings contain a few instruments in unexpected places if you're used to GM. For example, Clavinet is included with the Piano group, and Dulcimer with Chromatic Percussion. This is the way it's set in the official XG map, so I've leaving it as is.
Also note that if you're running on actual XG software or hardware, choosing an option under Drum Kits will set that staff to drum output, regardless of whether you're on channel 10 or not. So be wary if you do this and then go back and listen to the song with GM, as it will probably result in an atonal hurricane.
EDIT: Fixed a mistake in the tree; I had all the Ethnic instruments listed in Synth Effects.