About adding more stuff regarding mobs; The Apocalypse mod has something called a "full moon". Apparently it can feature something called a "siege event", where special mobs spawn to get through your defenses, in addition to normal mobs. It also adds increasing difficulty depending on playtime, but you may want to play around with the configs for that. Didn't actually experience it myself yet, just read about it. But sounds more interestening than Bloodmoon, for me at least.
About the engine sound, I'd suggest lowering the volume, without disabling it completely.
Aside from that, sounds like a great pack, just what I've been searching for. Now to find some people to play with...