My choice is clusivity for English. If I'm talking to you in English, and say, "We're going to the party next week," it is not obvious whether I'm referring to myself plus you, or myself plus other people excluding you. In some languages you can make this distinction.
The other, really obvious one for English would be a proper gender-neutral pronoun that is not 'it' or 'they'. English used to have one, ou, and I personally would like to bring it back.