I think it's probably possible to create an interesting fusion of the bardic inspiration and spells, and perhaps one that lets the bard choose how much to concentrate on spells proper vs. the various magical or quasi-magical abilities they have long had.
For example, suppose one could semi-permanently sacrifice a spell slot to gain the equivalent of a bard song. (This is obviously a common structure in many games, but in 5e it could be a unique way to model the bard's fluid relationship with the notion of arcane magic.) The bard song essentially becomes an at-will or permanent ability, although some may be activated and/or be enhanced by expending bardic inspirations.
The main thing in my mind is to make sure these abilities wouldn't behave like one-off spells. Something like "spend one bardic inspiration to make a suggestion" is rather pointless if one could just cast suggestion instead. (I could perhaps see the occasional exception if one wanted to trade a high-level spell slot for the ability to use a low-level spell limited by bardic inspiration rather than spell slots. Mostly that'd be boring, but with the more limited slots in 5e perhaps it wouldn't be quite as underwhelming as it was in 3e.) Contingencies, sharing proficiencies, bard-themed "metamagic", highly contextual inspiration (e.g. a ranger fights a favored enemy, a wizard casts a signature spell, a character suffers the mechanical effect of one of their flaws) and things of that ilk sound better to me.
In this approach, a high-level bard that has traded in all 3 7-9th level spell slots but kept the rest would likely play quite differently from one that has a expended 5 or 6 lower level slots, but kept room for big spells. A multiclass bard would still have sacrificed the appropriate slots, but the overall progression of slots would be unchanged.
We could also use this to do most of the heavy lifting on subclassing. Some abilities might focus on manipulation or combat. Some might focus more specifically on the means of inspiration, so that there is room for music or poetry as opposed to something more plain-spoken. If one wanted a bit more differentiation, I think we could have class abilities that grant different side-effects whenever one spends bardic inspiration for any purpose. The vanilla bard might grant a small skill bonus, the skald might incite the party against an enemy (grant extra damage), and the warlord might allow some minor healing.
Just thinking out loud.
For example, suppose one could semi-permanently sacrifice a spell slot to gain the equivalent of a bard song. (This is obviously a common structure in many games, but in 5e it could be a unique way to model the bard's fluid relationship with the notion of arcane magic.) The bard song essentially becomes an at-will or permanent ability, although some may be activated and/or be enhanced by expending bardic inspirations.
The main thing in my mind is to make sure these abilities wouldn't behave like one-off spells. Something like "spend one bardic inspiration to make a suggestion" is rather pointless if one could just cast suggestion instead. (I could perhaps see the occasional exception if one wanted to trade a high-level spell slot for the ability to use a low-level spell limited by bardic inspiration rather than spell slots. Mostly that'd be boring, but with the more limited slots in 5e perhaps it wouldn't be quite as underwhelming as it was in 3e.) Contingencies, sharing proficiencies, bard-themed "metamagic", highly contextual inspiration (e.g. a ranger fights a favored enemy, a wizard casts a signature spell, a character suffers the mechanical effect of one of their flaws) and things of that ilk sound better to me.
In this approach, a high-level bard that has traded in all 3 7-9th level spell slots but kept the rest would likely play quite differently from one that has a expended 5 or 6 lower level slots, but kept room for big spells. A multiclass bard would still have sacrificed the appropriate slots, but the overall progression of slots would be unchanged.
We could also use this to do most of the heavy lifting on subclassing. Some abilities might focus on manipulation or combat. Some might focus more specifically on the means of inspiration, so that there is room for music or poetry as opposed to something more plain-spoken. If one wanted a bit more differentiation, I think we could have class abilities that grant different side-effects whenever one spends bardic inspiration for any purpose. The vanilla bard might grant a small skill bonus, the skald might incite the party against an enemy (grant extra damage), and the warlord might allow some minor healing.
Just thinking out loud.