I am very happy to see this being talked about. I would be very much in favour of having a single tool that is able to format Swift code, and I feel like it shouldn't be integrated only with Xcode. I know people who use VS Code to write most of their Swift tools (I do that myself from time to time), so seeing that this project is starting out in the open is already a good step.
I also feel like if we get the Swift team to care about these tools, their quality will definitely increase. I've found a couple of bugs in the third-party solutions that are powered by SourceKit under the hood, and that's because SourceKit wasn't meant to be used in that way and the maintainers are not really checking their API changes against these tools. Using SwiftSyntax and including the tools required in the Swift project, would definitely raise the quality of the tooling in my opinion.
I spent the last couple of weeks formalising our internal Swift style guide, and trying to get most projects to adopt it. It has been difficult, and this would have been much easier if the tools were included in the IDE (or available as plugins for the most popular ones).
I think it's pretty much agreed that including a formatter in the Swift project is a good idea and I honestly would love to see this pitch move forward to its full extent: enforce the code style everywhere. Sure, people will dislike it at first, but looking at the big picture I think the time gained from formatting code manually (or with tools, those still takes times to install and integrate) is something that we should strive for. On the other side, it's also true that tools should help developers, and we don't want people to waste time in fighting the formatter all the time.
I am definitely +1 this pitch, and happy to help contribute where I can.