The third eye suggests that we may soon see a more powerful, and possibly more evil, incarnation of our familiar Stephen Strange.
The longer answer is that the appearance of the third eye evokes other characters and powers from both within and outside the movie.
There is the obvious Sinister Strange, a character who has achieved absolute power and has been corrupted completely, and who at one point in the film reveals a very strange extra eyeball on his forehead.
This horror movie–style parting scare could also be evoking another Strange doppelgänger, known as Strange Supreme, from the Marvel animated series What If...?
In that episode, Strange loses Christine instead of injuring his hands in the car crash from the first Doctor Strange film (Rachel McAdams).
The desire to resurrect her drives him to keep absorbing powers from other beings.
Finally, the third eye evokes the Eye of Agamotto, Strange's magical pendant that once held the Time Stone in the Marvel Cinematic Universe.