Well, I originally wrote "Esperanto", but then I feared the comments would devolve in Ido vs. Esperanto vs. Interlingua. So I switched it to "conlang", but then I feared we'd get stuck in jokes about Klingon.
My main concern with a global conlang would be internal regularity/ease of learning. Regular grammar, regular spelling. Given a truly global scope, trying to make it intuitive to everyone is a lost cause. To learn from the (really very minor, considering) mistakes Esperanto made, I'd make sure to add: no diacriticals, and don't gender-tag words, or at least make neuter the default interpretation.
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My main concern with a global conlang would be internal regularity/ease of learning. Regular grammar, regular spelling. Given a truly global scope, trying to make it intuitive to everyone is a lost cause. To learn from the (really very minor, considering) mistakes Esperanto made, I'd make sure to add: no diacriticals, and don't gender-tag words, or at least make neuter the default interpretation.