I was discussing with @serailovesbagelset Theo’s potential future yesterday and it made me pretty convinced that despite the darkness within him and his connection to evil forces, he won’t ultimately follow the dark path.
His attachments to elves and to Isildur,who represents the faithful Numenoreans, and tension between him and Sauron suggest that Theo will choose a path of light.
However, I believe the strongest sign of his eventual turn toward good comes from a line spoken by his mother.
In this context, light clearly represents good. By choosing good, Theo will find the inner strength that overcomes the shadow.
It somehow echoes Galadriel and Sauron’s words: to find the light, one must first touch the darkness.
Galadriel’s struggle with her darker side (her pride, desire for power and even her romantic feelings toward Sauron) runs throughout the narrative. In her pursuit to destroy Sauron, she failed to see that her obsession was blurring the line between herself and the very evil she was fighting. At times, she seemed to be becoming Morgoth’s successor as she was about to use his means for greater good. However, despite these temptations, Galadriel found the strength to resist and sacrificed her darker desires in the name of love for Middle Earth. By doing so, she chose good and set herself on the path of becoming the Lady of Light.
Theo could follow this same path. While he is vulnerable, especially now that he is alone without Bronwyn and Erondir to guide him, his internal struggle will eventually lead him to reject evil, Sauron.
In the end, he will choose the path of goodness, guided by the light of someone who, like him, struggled with the inner darkness, someone who can find him in the shadow, heal the emptiness left by his mother's loss and lead him toward the light.
Having said that, I believe he will be the leader of the Southlanders who would refuse to join Sauron and whose people would be captured to be sacrificied in Numenor. He, as the great leader, will eventually become the founder of Rohan (having these parallels in mind to Theo>Théoden and Bronwyn> Éowyn).
What comes to the light =good, the final words of Celebrimbor to Galadriel take on even more significance in this context.
The elves must cast aside their pride and ambition and embrace the goodness within them, for that is the only way to defeat evil. This idea, as being previously suggested, parallels the journeys of other characters: Sam’s loyalty and goodness helped Frodo to complete his mission; Frodo and Bilbo’s compassion for Gollum played a key role in the destruction of the One Ring; and Aragorn’s pure intentions, stemming from a desire to fix his ancestors’ mistakes, helped bring down Sauron.