My suggestion/request would be to just have SE post a request request for proposal to replace A51 on the main SE blog, since SE has had years to do so, has not, and positive the community would find a better solution.
As such, the core philosophy (and bulk of the code) behind it is still more or less the same, but how proposals actually behave has changed dramatically.
We've also made significant changes to the lifecycle of sites after they're created, which you can find discussed on Meta Stack Exchange:
- https://meta.stackexchange.com/questions/260754/feedback-req...
- http://meta.stackexchange.com/questions/283663/its-been-a-ye...
All and all, this was one of many reasons I left, the largest being that communities in my opinion should be based around tags, not verticals; tags are in fact in my opinion how SO is run.
Again, thanks, and good luck!