Option 3: Thaddeus Young
Thaddeus Young has spent most of his 14-year career at power forward, though he did play some center with the Chicago Bulls last season. It's unclear whether he'll play center next season, though at 6’8, and with a 7’1 wingspan, his frame is identical to former spur Rudy Gay and is only an inch shorter than Eubanks. Therefore, I believe that he'll play at least some time at the small-ball center for the Spurs.
Unlike Landale, however, despite possessing a unique perimeter skill set for his size, he isn't exactly a stretch five. For instance, he's comfortable with the ball in his hands and can even get out in transition or take his defender off the dribble. Additionally, he can knock down the occasional jumper or 3-pointer, hit floaters in the paint, score in the post, and create as a secondary playmaker.
With Young playing center, he would also present some unique lineup possibilities. The Spurs could play him alongside players like Bryn Forbes, Lonnie Walker, Devin Vassell, and Luka Samanic, allowing for more shooting on the floor. He could also be a part of the closing lineup due to his defensive versatility.
There are some downsides to playing Young center, however. For instance, he would provide only subpar rebounding and rim protection compared to the likes of Eubanks and Landale. Additionally, while Young is a versatile scorer and a good passer, he needs the ball in his hands to have an offensive impact whereas Eubanks and Landale don't.
Ultimately, I feel that San Antonio will continue with Eubanks at the backup center spot, although Young will also likely be used in certain matchups and to close some games. Eubanks gives the Spurs an athletic big man who blocks shots, rebounds, sets hard screens, and finishes well around the basket, while Young's versatility will be useful against certain matchups and in crunch time.