I came up with a pretty interesting idea. Everyone can make any unit of any kind transform into another unit of any kind. Also, for challenges, you could make the unit perform an action once they finish...maybe they create a big shockwave. :)
Ye, a basic morph ability might not be that hard, but setting everything properly and especially getting the visuals to look nice could be interesting.
What about a spell that brings dead enemies back to life temporarily but as your allies? That could be a big challenge.
That would require all affectable units to be properly set up, because there is no way to get access to a dead unit without increasing its death time. Also as far as I know, there is no data only way to revive a dead unit or spawn a unit depending on the type of another unit (unless you want to separate each unit type with its own validator xD)
So I doubt, this will be successful as an exercise, and it is way too specific. No room for variation at all.
In that case, I would not limit it to vikings (most likely you did not mean to limit the spell to one unit anyway), there are many air units, which are suitable (overlords dropping baneling bombs ftw :D)