In thinking about it some more... making bards half-casters, or even non-casters, but increasing the number and type of battle hymns and the versatility of the bardic inspiration die. Or possibly making bardic magic work like the artificer spells do. LU artificers very clearly are using technomagic, thanks to the fizzle die--they're not just casting spells and saying that it's an item that's producing the spells, but using a (very breakable) item to produce a magical effect. A lucky artificer could use a device to cast a spell many, many times before it broke.
I'm not sure what could be given to the bard to make up for the lack of casting, though. Maybe allowing them access to a tradition or two of martial maneuvers, more tool proficiencies, and a higher cap on expertise dice.
I'd also bring back a "chance to learn spell" for wizards, and bring back "spheres" for clerics, and say you can only cast spells from the spheres your god offers (or have much smaller cleric spell lists but give gods lists of extra spells they grant in addition to the extra spells granted by the cleric domain). I know this is all very second edition, but it would cause the PCs to have extremely varied spell lists rather than the, let's face it, common lists of spells that everyone picks.