Late to the party here.
Personally I think Velanna could be an interesting cameo in Inquisition. Since she was originally planned to be in DA2, having merged with Justice, I think that if you killed Anders in DA2 that Justice could perhaps somehow find Velanna and merge with her. Instead of justice for the mages, Justice becomes an embodiment of elven justice. Once he merges with Velanna, knowing her strong feelings, she'd probably be plotting something against a human institution and become the next Anders.
This could make Velanna an interesting NPC when the Inquisitor is dealing with the Dalish. Side with the elves in some situation (you know its going to happen) and you might find yourself an incredibly brutal (but powerful) ally. Side elsewhere or against them and she would make quite the interesting boss battle (or mini-boss).
In-game she'd probably be more agreeable towards an elven PC (given the elven Inquisitor is Dalish), neutral towards a dwarven/qunari PC and hostile to a human PC.
I find her dialogues with the others amusing (especially Sigrun) and think she would do well with the whole Justice arc. If Anders lives, I can see another spirit inhabiting her. Something similar to Witherfang and Zathrian, or perhaps even a more gentle entity that changes much of her more abrasive, angry personality.
Plenty of room to develop her further, so I wouldn't mind her in Inquisition.