I would like to see the trilogy kept, sort of.
But what I would do is break up the DM's Guide into a general GM's guide, and one that is more attuned to the setting in question (and combine that with the monster manual).
I mean, flipping through the 1st ed AD&D DMG you have stuff like NPC personalities, qualities of gems, mental illness, discussion of alignment, how monsters/dungeons adapt to PCs coming back after a week, tips on wilderness or underwater adventures. Just sort of general advice.
OTOH, you have more game related stuff that is probably more tied into the setting of the game you're playing. Nobility, types of mercenaries, magic items, commentary on spells, etc.
I would throw all that in with monsters into a book aimed at GMs for a given setting type.
That way you could basically make the books modular. Basically WOTC would publish alternate or parallel PHBs and DMGs that were attuned to a given setting or theme.
Like how 1st ed AD&D had Oriental Adventures - basically it was an alternate PHB. They also had something similar for Greyhawk and Dragonlance.
That way a player would only have to bring one book (the one with his class in it) to the game. And the DM would also have a corresponding book with both information relevant to the campaign as well as monsters. (The general GM advice book would probably be able to be left at home).
** I would consider another magic using class - my preference is for a Witch - as it provides another clear fantasy archetype and also, in some ways encourages female participation.
That has always been something that I've found D&D to be inexplicably lacking. It's common in basically all the source material that D&D was based on, from mythology to fantasy stories