My personal thoughts below do not necessarily reflect those of BronyCon:
I think that the move is not so much a change as it is a recognizing of what's already been happening for years. Other content and cross-overs have been a hugely important part of the fandom since day one. Remaking meme's with ponies in it on the /b/ board before the Great 4Chan Wars. Making a Fallout x My Little Pony crossover fanfiction. Recreating famous movies with ponies on YouTube. Incorporating pony GMOD (and beyond) characters with Team Fortress, FNAF, Skyrim, etc., characters for transformative media. Having an SFM Ponies panel at BronyCon EVERY year. PONIES:The Anthology mixing in other pop culture and internet culture in-jokes and references with our own in-jokes and references.
Folks, BronyCon isn't really changing anything. They're just opening the door to allow us to express ourselves more fully as bronies. I'm a brony, but I'm also a fan of Disney, anime, video games, RPG's, American Football, and so much more. When more of your interests are represented and tied-in to your core interest of ponies, you see a greater engagement and appreciation of the event.
Besides, it's not like they aren't going to invite pony guests anymore. Plus, we've already HAD non-pony guests before. Cathy Westluck is better known for her work in anime than for ponies. John De Lancie is DEFINITELY better known for being the enigmatic Q from Star Trek: The Next Generation. Even Ian Corlett, who we had a couple years back, is famously know as the friggin' voice of Goku from the Dragon Ball series! BronyCon is probably going to focus on helping it's guests and panelists and musicians and vendors (and attendees) explore and celebrate all of what they do, not just a fraction of what they do.
BronyCon exists for it's attendees and the community. ALL of it's attendees and community. Have faith in the con and the fandom. It's not fair to write off the entire community if everything isn't exactly as you foresaw it or want it to be.
Also, I've seen a lot of people making the assumption that busloads of "outsiders" were going to be trucked in, invading "our space" at BronyCon. FIRST: This expansion is aimed at satisfying more needs of our attendees and attracting bronies on the edge of deciding to go. It is not so much about attracting those that do not like ponies at all. I know plenty of people who enjoy ponies and gaming, but decide to go to MAGFest or PAX East. By doing this expansion, we give reason for these conflicted bronies to choose BronyCon. SECOND: If there were outsiders coming in, it's completely against the nature of our entire community to shun, be afraid of, and want to repel "outsiders." Being a brony since 2011 doesn't make you any more of a contributing member of the community than being a brony since 2017.
actual personal opinion time: I think that it's a good thing, just so long as we keep in mind our focus and listen to how our attendees feel and respond. And right now, there's a lot of fear-mongering over losing our identity and getting rid of PONI altogether. These fears are pretty unwarranted, as BronyCon has stated many times that it is still pony-centric and will be incorporating SOME different properties into the event. If the percentage of pony were to slowly dwindle year after year, then yeah, no bueno. Rise up, and all that. But this is year one (or year six really, as per my above argument) and they haven't even made any details public yet. Please continue to express your concerns calmly, though. I know that BronyCon reads it, and I know that they want your feedback!