Speaking of Bleach, did you read the new LNs and short stories? There are two that came out after the manga ended which are pretty interesting. You can find translations and summaries here:
https://www.reddit.com/r/bleach/comm...fmh/?context=3. Happy reading (if you're still interested in Bleach).
Anyway, there are times when one would want a certain villain to join the good guys' side. If Mashima only does it at those times and with only those characters, it's still fine.
Mashima likes to reuse Plue. I find it fine, though. And he did manage to make them different across mangas even though he used the same name.