There are a few possibilities:
1) Jumpfail's friendship guide interpretation
2) A type of divination similar to Luna's dream walking, since that would enlighten you, particularly during the moonlight hours.
3) A more combative interpretation could be a pony who is good at offensive spells like a moonlight laser or something, though that one is stretching the idea.
4) A more niche idea would be a Pony that can tell what the phase of the moon is at all times even during the day.
Personally, I'd go with one for character interaction or two for actual usefulness (either dream therapy or spying on your enemies dreams).
Actually, dream therapy sounds like a great idea, just have the Unicorn need special preparation to do it (meditation on who she targets or touching them with her horn if she has direct access to them) and only for one person at a time instead of Luna's "I help safeguard the dreams of all ponies" thing since she's not an Alicorn. On patrol, she can still help due to the basic Unicorn abilities of Telekinesis, Teleportation, and general spellcraft, but have her main role being that of a pseudo therapist for the team.
If you give her a backstory like you did Scalpa, then make her an actual licensed Therapist who used dreamwalking as a part of her toolkit. That is what I'd do if I was writing this.
*Note: I don't see why you can't have the friendship guide interpretation, and then have them also use their good understanding of how to help others lead them to becoming a Therapist and learning dreamwalking from Luna or something. No reason for the two ideas to be mutually exclusive.