Addons such as Northern Sky Raid Tools will allow players to create their own alerts, within Blizzard's frame work.
Though the specific API changes are quite involved, the takeaway for players is that they can now create custom auras anywhere on their screen, similar to WeakAuras. The cooldown manager already allows players to track most of their spells, though it lacks customization. With these changes, addons will be able to expand on that, with more free reign to separate all of your tracked buffs on one side of the screen and debuffs on the other, or make one important buff much larger than the less important ones.
These API changes allow addons to expand on what the cooldown manager's buff tracking already offers.
Blizzard has helpfully added a series of pre-built categories to help addons handle this behavior, so they can create distinct visual groups for crowd control effects, dispellable spells, or big defensive buffs, but players will also be able to search by spell ID to create individual trackers for specific auras... though with an important stipulation.
Only debuffs on enemies can be tracked. This allows players to create DoT trackers and pandemic warnings for their spells, but will not let them to look up encounter mechanics or enemy player buffs in PvP.
Only buffs on allies can be tracked. This allows players to create a distinct alert for external buffs like Power Infusion or even allow a Priest to add an alert when their ally uses their own cooldown to time it with, but will not allow them to create custom trackers alerting players when they receive debuffs like Alleria's Silverstrike Arrow. The Exhaustion debuff is a specific exception to this rule.
All auras can now have custom audio alerts assigned to them, even aside from the two rules above. This means you can still have an Airhorn when targeted by an encounter-specific debuff, even though you won't be able to create a visual tracker for it.
All in all, this functionality is very similar to what could previously be accomplished using WeakAuras, though much more limited in scope. Players won't be creating complex alerts with multiple triggers or compound logic, but the more controlled set of rules allows players more visual control over the things they normally see on their screen already, while still letting Blizzard reign in computational problem solving. Unfortunately, this functionality cannot be duplicated within the game's native user interface beyond what the Cooldown Manager already offers, so it does work against the concept of "leveling the playing field" between those using addons and those who do not.
Reloe explains how his addon will make use of these changes.
While the video above relates these changes to the Northern Sky Raid Tools addon, it's a good visual explanation of how they work for players. Undoubtably, there will be many options from other addon authors as well.
