Is it weird that out of all the DBZ movies he wrote, the ones featuring Broly are the ones he seemed to care about the most? While none of his DBZ movies are actually good movies nor are the Broly ones the best DBZ movies (not even if you discount BoG and RoF for not being written by Koyama), they are the only ones where the OCs actually do more than just fight aside from WotD and I think he only gave a shit with that one because it was intended as the last one. I find that a lot of these TV show tie-in movies tend to subvert the demand of needing to keep most of the characters static to stay in line with the demand to keep them similar to what they are currently like in the show, by introducing OCs that will get the real character development and drama so that the movie can still have some semblance of purpose beyond just drawing a crowd from the existing fanbase. But the Koyama Dragon Ball movies don't really do that, the OCs in those movies are basically just villains to beat up with minimal development. Garlic, Wheelo, Slug, Cooler, 13, Bojack and Janemba pretty much just show and get beaten up the heroes with little to no meaningful interaction with anyone beyond that. WotD actually introduced a heroic character for the characters to bond with and a villain that tried to trick the heroes more than just fight them, but that's the exception.

With Broly though, not only does the villain Paragus try to fool the heroes, he has a proper backstory and motivation. And because the heroes don't know for sure that Broly and Paragus are villains, they actually do get to have conversations with them. Not that Broly himself seems all that keen on doing anything other than fighting but there is at least a little more to the movie itself. Then in the direct sequels, there is the stuff with the village and that rich dude, a lot more than you'd expect. Again, not good movies or even really the best of the series, but certainly shows signs of the writer being more interested.