Remove Duration as a stack, finite removal
Proposal 3: “Pulse” > 5 stacks of duration.
Now that duration is considered a stack, we have another trick available – one that solves the scaling problem. When you apply a 8th stack of a duration-based condition, 5 stacks get consumed and you apply a special effect. Now having 5 mages burning the same target is actually useful, since the duration converts to a burst of damage if the duration gets high enough.
Assume we use 2s burning for a stack. You get burning on a boss up to 16s. When you cast the spell that pushes it over the top, 10s of that gets applied instantly (perhaps with a 25% penalty to total damage?), and now you have 6s of burning left.
The penalty to burst damage, along with the high duration requirement (for non-elementalists, anyway) before it triggers is intended for balance purposes so that condition damage doesn’t become the new burst damage.
Take all 3 proposals together, tweak it with testing and experience, and I think condition damage becomes viable again… and more importantly, it’ll be easier to STAY fixed because the mechanics once again support scaling on both the application and the removal sides.
Oh, forgot to add one part:
For stacking duration conditions that aren’t damaging ones, the burst should be some special effect related to the condition.
For example, overstacking chill procs an immobilize.
Stacking weakness strips several stacks of might and/or removes unshakable stacks.
Stacking blindness gives a short-duration debuff that prevents the target from critically hitting.