Are there any plans to release Archfiends for the new enemies?
Short answer: Not yet, no.
Short runs of games aren't very profitable, and this industry's profit margins are pretty small to begin with.
The number of Harrowood+Lycanthropes Archfiends we could sell would be a percentage (if we could get it to 80% that'd be really amazing - I'd actually be surprised if it hit 50%) of the copies of Sickle Moon we sold, which in turn is a percent of the copies of 2nd Ed. This is one of the reasons expansions of expansions don't happen much. There's an automatic limiter in place.