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diablo 2 has always been like that , and aslong as you know how to use your mouse wheel it’s very quick to go from one ability to the other.
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diablo 2 has auras , yes auras , something that diablo 3 doesn’t have. being able to do what you are asking for would be the same as being able to use multiple auras at the same time.
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it would make the game even easier, which for most isn’t something that should be done, scrolling your skill as quickly as possible in a hurry before you could die in HC is one of the things that define what HC is. (same for thinking ahead in order to refresh your buffs , holy shield etc…)