Aperita Arcana is a Monsters & Magic supplement by Ebon Gryphon Games that expands on the core M&M rules by adding new character classes, new advancements, new spells and spell lists, rules for ad-lib spells, and much more. I warmly recommend this supplement, even if M&M is not your go-to system, if only for the excellent GMing advice scattered throughout the supplement, and for all the material that can be salvaged irrespective of your gaming system (e.g., the advanced ‘language’ rules).
The nice thing about Monsters & Magic is that character classes are not simply a bland collection of die modifiers, hit dice, allowed armour/weapons, or numbers of spells per level. In M&M, each character class is fleshed out by a series of traits and advancements that it may use but, again, not as mere 'plus sometings' but as narrative tools to enliven your role-playing experience (and kick ass — that's the magic of the Effect Engine). This is why, even though I used to hate new classes when I played AD&D, I have really enjoyed the ones in Aperita Arcana: since a class a mostly a collection of traits, they add wonderful role-playing potential in terms of what effects the player may try to attain.