I'm trying to remember the title of a story i read where a man is hired to research primitive aliens by being altered surgically or genetically to live as one of them. The only catch is he can never go back to being human. After several different body changes, the main character is told he can be modified one last time, and he can pick what he wants. He chooses to become a being with the best aspects of all the different aliens he experienced.
I think this is A Miracle of Rare Design by Mike Resnick, which was also the subject of the question Book about a man sent to multiple planets, surgically altered to fit in with the aliens.
The protaganist is Xavier William Lennox who is an author of alien travelogues. As you say he does undergoes surgical modifications to make him look like the aliens he is researching.
At the end of the book he is told:
“I've just come back from the Department's headquarters on Deluros VIII,” she continued. “Our budget has been cut—well, re-directed, actually—and your particular project has lost its funding.” She paused, trying to discern a reaction, but finding none. “It makes very little difference anyway. As I told you after your last surgery, it is the opinion of Dr. Ngoni's staff that you can only survive one more transformation.”
Lennox merely stared at her, as if waiting for her to continue.
“In appreciation for your unique services to the Republic, they have set aside enough money to pay for that transformation, but since we have no assignment for you, you are free to choose any race you want—or to go back to being a Man.”
His response is:
“Dr. Ngoni says you can choose any oxygen-breathing race except for the Domarians and the Mollutei.”
I KNOW WHAT I WANT.
“Good. What is it?”
He entered a number of commands, and the machine slowly pieced together a holograph in response to them.
“I've never seen such a thing before,” said Nora, studying the image. “It's quite beautiful.” She stared at it intently. “One might almost say awesome. What planet does it live on?”
NONE. Then he added: YET.
“Is it your own creation?” she asked, surprised that she wasn't more surprised.
YOUR CREATION. MY DESIGN.