Arguably, I’d say aff/demo/destro should get collapsed into one spec. With options in the tree to design out as being more dot heavy, demon heavy, or self-cast heavy in your actual damage rotation. With some interplay for benefits to being a little of each.
That, or one of the specs is interwoven with the other two for a similar effect.
Then, tank AND healer specs get added to warlock.
In my experience, the problem with warlock (gameplay wise) is it feels like the design is too difficult between the three because they’re all similar but trying to be different. So some differences exist purely to be different. A ‘merging’ would simplify that down, while giving some variety to their designs. And by lessening the amount of specs in it, opens the opportunity for tank/healer specs to be added.
I wouldn’t want to replace aff for a ‘life stealing’ healer, but would love a warlock healing spec inspired by it.
I wouldn’t want to replace demo with a tank spec, but a Metamorphosis based tank spec sure would be cool.