Problem
Currently, spell cooldown icons allow considering GCD when calculating the icon duration. This is helpful when having a group ordered by icon duration (Acting as a Rotation Helper) to treat spells that will come off cooldown sooner then the GCD the same as others that are already off cooldown.
However, casters still have a similar issue with cast time. Example:
For a Demonology Warlock, casting Call Dreadstalkers (CD) takes priority over casting Hand of Gul'dan (HoG). A simple way to have a "Rotation Helper" is to put both spells in a Group ordered by Duration > ID, and have CD before HoG.
Problem happens during casting a spell (e.g. 2 seconds cast), when CD would come off cooldown in 1s, as the Group will show HoG first (As it doesn't have a cooldown), while the next spell in the priority list is HoG (As it will come off CD by the time casting finishes)

Suggestion
Allow Icon duration to be affected by Cast time, similar to how they cand be affected by GCD now.
This means that, during casting a spell, all Spell Cooldown icons should have a duration of Max(Icon_Duration, Current_Cast_Time)

Alternatives
The only other alternative is to code all Spell Cooldown icons in LUA, which is very cumbersome.