Gunn lets it be known tha he didn't include Baby Groot in Guardians of the Galaxy 2 as a way of selling more merchandise for Disney, but rather Baby Groot fit with the natural progression of the story. Gunn also offers up at one time he did have plans to include an older Groot when he had the idea to have Guardians of the Galaxy 2 take place years after the first movie. Marvel also recently released the first image of Baby Groot. See below.
Via James Gunn's Facebook:
This headline really couldn't be any more true. To be honest, when I was first working on the script for Vol. 2 I was planning on it being years after the first movie and for little Groot to be a grown Groot. But there was something missing. First of all, I thought there was a lot of development the group needed to go through as a group – and it would be a shame for the audience to miss it. And, secondly, for whatever reason, Groot just wasn't working. It was then I came upon the idea of having Vol. 2 take place very shortly after the first film and for Groot to still be Baby Groot, with quite a lot to learn. Even though I had already long-ago-decided on the other characters involved, this change opened up the whole movie for me and it suddenly all came together. I fell in love with the script for Vol. 2, and I felt like we were creating something very special.
When I approached Marvel Studios with my idea for Vol. 2 I was afraid of the involvement of Ego the Living Planet – a rather, uh, broad character in the 616 universe of Marvel Comics. I was afraid of the inclusion of Mantis and Ayesha, and more prominent roles for Nebula and Yondu. But I was even more afraid of approaching them with the concept that Baby Groot, and not Groot, would be a member of the Guardians of the Galaxy. Yes, perhaps it seems like a hole-in-one idea now, but Groot was the most popular character in the first film, and sometimes when something works studios are more than a little reluctant to change.
To my happy surprise, my partners at Marvel were more than supportive about the big swings in the story of Vol. 2. They seemed to instinctively know, like I did, that what matters in the Guardians franchise is to keep giving people the unexpected…
All that said, I'm not an idiot. I knew if Baby Groot worked, the world would want Baby Groot toys and figures and plushies. But that certainly didn't seem like a certainty when I was alone in my office conceiving of a story, and it most definitely was not the driving force of the decision. I, like so many of you through the trailer alone, had fallen in love with the little dude – even though at that point he was only in my head.
"Guardians of the Galaxy Vol. 2" has a May 5, 2017 release starring Chris Pratt, Zoe Saldana, Dave Bautista, Karen Gillan, Bradley Cooper, Vin Diesel, Michael Rooker and Sean Gunn. New cast members include Pom Klementieff, Elizabeth Debicki, Chris Sullivan and Kurt Russell.
Set to the all-new sonic backdrop of Awesome Mixtape #2, "Guardians of the Galaxy Vol. 2" continues the team's adventures as they unravel the mystery of Peter Quill's true parentage.