Mistwell said:
I don't care much beyond whether the interpretation is one reasonable way to read the feat, and that it is balanced. I think most DMs and players handle new rules that way.
I'm not convinced it is balanced. Even with my interpretation.
Bard 2 / Sorcerer X
This Sorcerer took this combination in order to get the Cure Light Wounds spell.
Normally, he can cast it once per day. With my interpretation, he can cast it twice per day. With your interpretation, he can cast it 5 times per day.
This single feat increases his Bardic casting ability by a factor of about 2.5 times with no significant cost to the character (i.e. the 0th level Sorcerer spells are generally worthless most days anyway and even if he needed them, he could use higher level Sorcerer spell slots to cast them). It also increases his highest level Sorcerer casting ability by a factor of about 1.75 (or more).
What other feat allows a caster to cast his highest level spell for a given class 5 times as often as before?
Compare this to the Spontaneous Casting feats for non-spontaneous casters. They get a few select spells they can replace. They do not get all spells from all of their classes, nor do they get to replace spells in all of their slots. Balance-wise, your interpretation of Versatile Spellcaster is stronger and broader than the Spontaneous Caster feats.
Balance is not just about potency. It is also about flexibility.
An 18th level Sorcerer who can cast 5 or 6 Wish spells is nearly as potent as a 20th level Sorcerer. This feat allows him to not pay his dues (i.e. 2 more extra levels). He can up each of his ability scores by 5 with Wish without having to wait until 20th level to do so.
So like I said, I am not convinced that the feat is overly balanced in the first place. Cool ideas do not necessarily equate to balance.