I seem to remember reading that apparently the D1 tools at Bungie were a complete nightmare, things like it taking 24 hours to load up one of the zones to work on it for the artists, things like that. Basically, their tech was not scaling well and inhibiting their ability to make new content easily. Perhaps that's partly behind the full sequel decision and they have better tools to work on the sandbox.
Citation needed but I can't remember at all where I read/heard that.
"People who worked on this project say that one of Bungie’s fundamental issues over the past few years has been the game’s engine, which the studio built from scratch alongside Destiny. Four sources pointed to Destiny’s technology—the tools they use to design levels, render graphics, and create content—as an inhibiting factor in the game’s development.
“Let’s say a designer wants to go in and move a resource node two inches,” said one person familiar with the engine. “They go into the editor. First they have to load their map overnight. It takes eight hours to input their map overnight. They get [into the office] in the morning. If their importer didn’t fail, they open the map. It takes about 20 minutes to open. They go in and they move that node two feet. And then they’d do a 15-20 minute compile. Just to do a half-second change.”