Given that the Barbarian's Rage ability is the centerpiece of the class, giving full Rage progression via a feat seems overpowered. It reminds me of the people who try making Spellthief 1/Wizard 17 builds in the belief that the Master Spellthief feat will let them steal 9th-level spells (hint: it doesn't. While they can theoretically steal 9th level spells, they've only got one spell slot to squeeze nine slots worth of spell into. Because there isn't enough room for the stolen spell, it instantly goes right back to the original caster. You'd need at least nine levels of Spellthief to hold a single 9th-level spell at a time).
Since Rage is too powerful (unless you count each Bard level as 0.5 levels towards Rage?) and advancing Trap Sense is boring, perhaps progressing the Barbarian's Improved Uncanny Dodge class feature would suffice? It'll help keep your lightly armored character from being sneak attacked.
Alternately, you could try allowing a Bard to spend uses of their Bardic Music ability to enter or extend a Rage.
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"Anyone can be a winner if their definition of victory is flexible enough." - The DM of the Rings XXXV
Since Rage is too powerful (unless you count each Bard level as 0.5 levels towards Rage?) and advancing Trap Sense is boring, perhaps progressing the Barbarian's Improved Uncanny Dodge class feature would suffice? It'll help keep your lightly armored character from being sneak attacked.
Alternately, you could try allowing a Bard to spend uses of their Bardic Music ability to enter or extend a Rage.
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"Anyone can be a winner if their definition of victory is flexible enough." - The DM of the Rings XXXV