In an Alternative epic level system thread in rec.games.frp.dnd, I had cause to examine the spell slots available to the various classes. I was able to come up with a reasonably close approximation to the spell slots available to all of the base classes. Examination of the various spell lists led me to the conclusion that the ‘weakest casting classes’ (paladin and ranger) could almost as easily be changed to use the cleric and druid spell lists with little loss. I have not yet fully examined the bard list, but a cursory look suggests that folding the bard spell list into the wizard spell list, then using that combined list for bards, would actually work out fairly well.
Further examination of the spell lists in the RSRD (which involved me building a 600 row spreadsheet to capture the numbers) indicated that where a spell appears in more than one spell list it is almost always at the same level. Most of the exceptions show up with the weaker casters (bard, paladin, ranger) lists compared to the primary caster spell lists, and as I’d found, using the primary list spell level and the modified spell slots tables ended up in the spells being available at approximately the same class level as RAW.
This suggested that it may be possible, and even feasible, to unify the spell casting system so all classes cast spells using the same mechanisms.