Assuming 5.0 will bring 3-4 new spells per job, we can hope this option to become available and more spell merging for selected few spells
However, it if were to be implemented, it would have to be optional, there is case where you don't actually want to go through your combo chain.
Mana regen on both DRK and PAL is one example.


On the other hand, many job could greatly benefit from a this feature allowing you, personally, to create your own combo.
For instance, SMN and BRD might want to put their DoTs on a combo system to save on slot as you simply chain both skills in most scenario.
But I stress the importance of this feature being optional as in the case of the DoTs or tank MP regen, you might want to have access to each spell individually. On top of that, making it optional can only be positive as anyone not enjoying the option can simply ignore it.

It would also be nice to be able to manually merge some spells that activate under specific conditions, for instance
Ley Line -> Between the Line
FireIV / Blizzard IV (since both can never be usuable at the same time)
Blood Weapon / Blood Price