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OK, In my opinion, in order to create a DPS class extension (and really healers and others also) it is supremly important to have mana management factored in.
In order to manage mana, the fight length is a huge factor.
I know an exact fight length is essentially impossible, however an estimated time till death for a boss based off perhaps a default value for boss fights, either per boss, or a generic 4minutes, that is modified by time in combat divided by % of boss health.
I know that every boss is different, some have multiple health bars, so to speak, as you kill them and they reincarnate or turn into a dragon or similar, and, ideally, this mod would be able to know for wach boss you're fighting what factors to consider for estimating time till encounter is over (yeah, that would be a better description of idea feature: time till encounter ends, rathar than time till boss dies)
However, even at it's most basic as first described, based off the target's health bar and time in combat, probably with some sort of mob_id check to make sure that the boss is tracked even if target has to change for cc or add burn purposes...

Take a fire mage as an example:
They need to balance fireball and scorch to maintain a mana ratio that allows them to have enough mana to spam fireball from 35% till death on the boss.
this is bolstered by using evocation to restore mana back up, assuming it's at a point in the fight this is practical.
So, the estimated duration of the fight directly decides how many scorches to cast to a fireball, which is what needs to be suggested.
And almost all dps specs need to know when there's, for example ~30 seconds left in a fight, so they know to pop any remaining 20s cooldowns.
There's a few other skills with different durations - some 30 second cooldowns, some larger, some smaller.

And from (again) a fire mages perspective, any fight finished with >0 mana is a fight you didn't do your best DPS in, as is any fight where you run out of mana before the boss dies (and I believe there are other classes with talents that allow them to do increased damage on a boss when it's health is below a certain %)

Ideally this would be incorpporated with a module/library that could identify 'burn phases' for bosses.

With the above, it would be childs play to write a spellflash module that allowed relative 'noobs' to outdps many seasoned hardcore raiders (or outheal, etc)