Remathilis
Legend
One time, during 3.5, we did a lark game where the rule was you couldn't use PHB classes or races. We ended up with a Crusader (Bo9S), Shadowcaster (ToM), Psychic Warrior/Soulknife (EPH), a Binder (ToM) and Warlock (CA). I want to say there were some prestige classes in the mix. I was the warlock. Everyone was 5th level.Bards should have their own unique subsystem, I think, for their equivalent-to-spells. Entirely based on sound and what sound might be able to do, and how the Bard uses voice and-or instrument to manipulate that sound. Their casting wouldn't be restricted by slots but by exertion limits per x-amount of time, with said limits rising as the Bard levels up; and their potential effects would fill a rather narrow band: sonic damage, sonic mind manipulation (charm, panic, etc.), sonic detection (e.g. getting magic items to resonate sound), and long-range sonic communication (e.g. Bard as radio transmitter!); along with legend lore and item knowledge.
All I remember was it was a disaster. The first time we got into combat, we realized everyone's abilities had different names, mechanics, and limitations. The DM literally had no idea what any of our characters could do, which resulted in a lot of "quoting the appropriate sourcebook" We got through a couple of small fights, but it fell apart quickly with everyone buried in books against some "boss monster" aberration from Lords of Madness, and we kinda gave up after it.
Now, the purpose of said game was to try out the different systems as most people had never had a chance in regular games to do so. But having so many competing systems (which referenced other systems, like psionics referencing spells) each with their own quirks, recharges, and rolls, was a massive headache. Say what you will about the sameness of 5e using spellcasting for 9/13 classes, at least it's easy to predict what a PC can do and what they are capable of!