These can be spells in the spell list.I explained part of it before
I would combine Bardic Inspiration into a resource that powers several class features.
One would be Bardic Inspiration as we know
Another would be Song Magic casting of wizard level sound based spells that lack concentration and are "uncounterable" by "spells".
Another would be Countersong, one of the few ways to counter Song Magic and allows the bard to counter and dispel thunder or sound based magic effects
Another would be Theme Song, which lets the bard tap into reality and play a character's theme song and give them fighter-level ability (action surge and second wind)
Another would be Dirge of Doom or something that lets you roll your die as necrotic damage a bunch of times rogue-style when you hit with advantage
The reverse would be the bard playing music to cast a cleric-level healing spell on a target
Another would be just like the Whisper Bard's Psychic Blades, added psychic damage to attacks.
Another would be a sort of magical Synesthesia Echolocation mixture where the bard plays music or makes a sound and it allows allies to see the sound to look behind walls and see in the dark.
A bard would get Bardic Inspiration, Song Magic, and Countersong automatically then choose which other bardic secrets they know.