Being able to add or remove bossbars would be great. Whether to add a bar to a normally non-boss mob, or remove one from a normally boss mob.

As for Passengers, I feel like it should be an included feature but my attempts to implement riders to my mob templates have failed.

Hmm. 2 very different concerns. I'll probably split one off into a separate issue.

The passenger one feels a more functional feature, and an easier one to implement. I can't imagine having an invasion without calvary.