Whilst I understood the lack of musicians in the original release of AOE3 due it trying to be simpler with less nuance in units, I think the game has evolved into its own niche on the unit front, which does of course musicians in the form of African UUs and Royal House Drummers.
With this in mind, I would love musicans to be embraced as a default unit type. During the whole roughly 1500s - 1800s timeline, literally all the civs in-game had musicans. To take a small perspective of it, we can look at the Europeans and see that Drummers, Fifers, Pipers and Trumpeters were all used with their respective armies and navies.
What I’d really love, is a total Command Unit - a purely support ‘crew’ of people consisting of a General/Officer/Leader with a Flagbearer and Musicians - acting as a single unit (in much the same way artillery crews do) and shamelessly stealing the mechanisms from Rise of Nations’ General/Patriot unit…
…but anyway I’m sidetracking myself. Basically Musicans should be available to all in various guises and trainable via the Barrracks/equiv and be expensive in cost but worth training multiple to extend their area of effectiveness.
Going back to another old game, ‘Cossacks: European Wars’ and its offshoots - your armies always felt completely having the drummer in their midst and just really helped with the immersion, with the drum beats following the mass of marching musketeers.