The spell Twilight finished

[ggg-2 3,553]

in magical mystery cure, it was stated to be starswirl's magnum opus, but what do you think the spell was intended to do?

[Courageous Thunder Dash 7,824]

I think it was intended to find the true destiny of somepony. 

[StarSwirlTheBearded 356]

By chanting the first part, Twilight switches around the Elements and the pony they belong to ("one to another, another to one").  Twilight's version of it essentially means that friendship is magic (very cliche IMO).  The completion also teleports her to the alicornification vision-thing.  However, Celestia later states that Starswirl (or is it Star Swirl?) did not understand friendship; thus, his version would have ended differently.  I don't know what the spell was intended to do.  Here are a few of my ideas:

• The spell would be able to give a pony its cutie mark.  Either it would speed up the natural process or the caster would be able to decide the mark, and hence, the pony's destiny.
• It could be a cutie mark switching spell, as was demonstrated in the episode, although probably more refined.
• Maybe it would be able to enable two ponies to switch minds/bodies.

We'll never know what was intended because it was probably never decided by the people who make the show.

[Rift enchanted 380]

Well it showed that at first twilight performed the spell incorrectly so it couldn't be a switching spell.

It's some friendship thingy, I don't know ask starswirl.

[Malinter 3,064]

Well, considering the spell can alter a pony's destiny. Maybe starswirl originally began researching the spell as a means of restoring Luna.

 

Nightmare Moon in essence corrupted Luna's destiny and Celestia alone couldn't cure her using the EoH so perhaps Celestia asked him to research other means of curing her (after all, why would Celestia want to wait 1000 years if a solution could be found sooner?), I figure Starswirl did figure out away but realised he himself could not finish the task, so he left it incomplete and Celestia began searching for a pony who would be able to complete it.

 

Twilight was able to restore luna without the spell (another sign of her magicial power) but completing the spell would change twilight's destiny (again ) up its completion.