There aren't really any restrictions to when you use a team LB, either. You just need to have enough gauge. It just so happens that it's usually up when the fight needs to have it up.

There's actually not that much difference between a gauge or a timer. PLD still has Sheltron up at fairly predictable intervals. With longer gauges, however, you introduce more possibility for variation, depending on fight-specific factors (uptime, damage mitigated, etc.) So it becomes less a matter of creating a timestamp list of boss actions and planning out swaps and cooldowns on a spreadsheet, and more about adapting on the fly while rationing out your regular cooldowns in-between invuln uses with good team communication.

Killshots are an interesting idea for dps players. Granted, not every fight has much in the way of adds.

For tanks, could you not just reward players for mitigating damage on themselves or a co-tank with their short recast cooldowns with more gauge? That way, the goal would be to try to build up gauge faster by intelligent use of single target mitigation tools.