It might also worth thinking about making the songs last as long as you have MP rather than on a timer as they have been so far. Make it like Foe Requiem used to be as that was somewhat of an interesting way of doing it
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The design of those abilities would to be altered so they could be toggled off (think SCH Aetherpact) ... the reduced cooldowns are to allow flexibility, as MP would now limit how often / for how long they could be used (i.e. their uptime). Note: Remaining cooldowns / activation costs are to prevent abuse (e.g. rapidly toggling the ability on and off to line up with individual big hits).
That would be how Troubadour works... I didn't want to alter BRDs personal gameplay too much though, with the exception of making the song cooldowns shorter so the BRD has more choice about which song to play when, but still requiring some planning / fight knowledge.
I'd suggested back when we first heard about the SB changes to come that they use increasing drain rates over use, which diminish back to the normal drain rate over time without use.
For instance, if I opened with Wanderer's Minuet, it'd start off depleting my MP more slowly than I gain it, but would rapidly decrease to a loss and a increasingly steep loss with further time, until it'd bottom out my MP after about a minute. So, I'd tend to swap around 30 seconds in, restoring MP at a decreasing rate over the first 15 seconds, then losing it at an ever-increasing speed again, until I'm forced to swap off again. So, by the time I finish my first song, the penalty on the first is gone. You maintain the rotation, but with an MP bar's worth of flexibility, allowing you to effectively bank duration and make more flexible use of downtime.
I would assume it'd be because the core ability or abilities (where mutually exclusive and/or alternated, such as through dynamic MP costs) would have MP drain costs, and that drain would end up cancelling the buff, such that you can refund the remainder of the buff, having effectively only wasted some portion of the initial activation cost.