Yaarel
🇮🇱 🇺🇦 He-Mage
What do you think of more limited casting, but allowing the bard to chose spells from ANY class?
For 5E, they still have their own list, but can raid from anywhere else. With the restricted spells and possible slots if a Warlock-like build, I think it might be okay and a fun option.
Designwise, a FULLCASTER class cannot lose its spells and the result be remotely balanced.
Start from scratch, and build up from there.
Rogue is a good chassis with an example of what a third caster can look like.
Even the Fighter with a Charisma version of Eldritch Knight might work well − while beefing up skill options.