i guess the obvious would be requiring the booster or the air tank, but there's also stuff like 'you need to shoot this switch to activate it but you need the missile launcher because your regular gun doesn't have enough range' or something...
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Eli
Mint
+1 on the range thing yeah, I once modifiede a weapon to make it's sole purpose to trigger a 'hit' at a range (on objects with 0 hp IIRC).
Don't forget that you can also set wether weapons are able to break blocks.
Don't forget that you can also set wether weapons are able to break blocks.
Serri
There is also the Mimiga Mask, since that can be equipped without having the item in your inventory. Not sure how that could really help, unless combined with a health up/weapon upgrade.