I understand what your saying but I also feel like removing flight for the sake of making your pack unique isn't the best idea.
If your removing flight or any other perk for lack of another word you need a mechanic that affects why your doing this. For example you remove flight because your pack is based in the 1700's and they didn't have flight or teleportation.
The only justification I need is that IRL jetpacks are horribly inefficient and those few designs that aren't are still incredibly dangerous. A flightless mod pack would purely be for the more masochistic modded MC players anyway. Nothing about being unique. I just feel like there are a few mods out there that become obsolete due to this jetpack paradigm. Players are really reliant on jetpacks to get around. Things like carts and boats are only considered as a novelty or super early game (speaking of boats there really, rails are mid game modded or vanilla).
Maybe I wouldn't get rid of flight all together but cruiseing around in a super efficient jetpack takes something from the away from me, I hope I'm not alone here.
Maybe such a pack could utilize Archemedies airships with a tweaked recipie for the balloon block make it utilize mid/lategame materials maybe even use that mod ME 2 used quadrum to make a gated material for it that sort of makes sense. Eg make a sort of high tensile aeronautical cloth that requires rockwool, canvas from IE and glue balls from tinkers or rubber from ic2. Maybe have it crafted in a machine. So players don't just rush for the bare minimum airship on day 2. Maybe make airship speed a tad slower too.
I also wished there was a method of changeing the functions of a items mechanics. To change some of the flight giving items to do something else useful. Like make it so the Thaumostatic harness creates a ghost platform rather then give creative albeit slow flight. Or jetpacks in general give the player a really boosted jump rather then full controlable flight.
I may prototype such a pack for my self but it's probably too much of an undertaking. I'd like to one day see a server that has a well built Railcraft network that's actually useful and not just an unused novelty. Or towns with large vast airship docks. Having things freighted around by rail.
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