I Will Ask You Questions - Now with MORE fantasy ecology!

Imagine that humanity adopted a new language as the international tongue of trade and science. This language has no homonyms, no words with more than one meaning, and all words are spelled phonetically in order to promote clarity. How would our sense of humor and poetry be different in this new language?

I have two immediate observations.

First, you did not specify that this new language would replace all language, merely that it would be the international language for trade and science. If existing languages remain in use, then it's highly likely that words in the new one will be similar to words in other languages, and so a good amount of humor would be based in multilingual puns.

Second, the condition that there are no words with more than one meaning will not last. This is inevitable. It's just easier to repurpose an existing word as a metaphor or to cover a similar-but-not-identical concept than it is to come up with a new one and explain it. This isn't even considering loan words - if the common language isn't sufficient to explain a concept but your native language has a word for it, you're going to find it easier to use the latter. For that matter, the phonetic spelling probably won't last either, because pronunciation habits will change over time. Over the long term, this language will be just as unwieldy as English is today.

Let's take the hypothetical a step further and assume this language becomes universal somehow. Other languages only exist as curiosities, they're studied by academics and a few hobbyists and that's it. We're still likely to find artists who feel that the language isn't sufficient to tell their stories, and so we more than likely end up with a Shakespeare equivalent who coins thousands upon thousands of words, many of which are simply adaptations of more familiar words, or perhaps taken from the now-dead languages.

So I feel in the long term, the constraints on this language will be eliminated, because language evolves through use and most people who actually speak it regularly won't be too concerned with preserving its unambiguousness. But that still doesn't mean poetry and humor would work the same way as they do now, of course; more likely they'd develop in a certain direction as the language got started and that would inform their eventual shape.

So what would the short-term effects be? Well, let's look at the language as it starts being adopted. Before we start metaphoring it up and adding all sorts of new words and ruining its basic structure, this language would primarily be used in contexts where we want as little ambiguity as possible. Laws, contracts, scientific explanations. So the first wave of art and humor in the language would probably be based on that model. Humor would be largely satirical, consisting of legal definitions or scientific analyses of things that don't really need them. For other forms of humor, it would generally be easier to use your native language, and you'd make the choice to use this one only if you felt the joke worked best in it.

I haven't talked much about poetry, probably because that's not really my strong point. On a structural level, I think poetry would be concerned with the rhythm of the words; even if the language initially has no rhymes, we already have plenty of examples in existing languages of writing based around flow, such as iambic pentameter. So I think this aspect would be emphasized.

So how would these things affect the long-term development of the language, if it eventually becomes "the primary language of everyone on Earth"? I feel like poetry would still be heavily flow-based even as the language adapts to have more rhymes. It would be far more important for two lines to have the same rhythm than it would be to have their endings sound similar, especially if rhyming ended up being rather uncommon. Most poets would find it boring to keep using the same rhymes over and over.

More generally, though, I think poetry and humor would borrow heavily from the languages that died out. During the period where the common language was greatly increasing in use but other languages were still actively being spoken, people would probably find themselves bringing over habits from their native language into the new one. So more than likely, over the long term such a language would end up being an amalgam of dozens of others. In fact, it's quite likely that our modern languages would end up heavily informing different dialects; perhaps even to extremes where it doesn't feel like the universal language really is that universal.

So overall: I feel that the defining characteristics you've noted would become less and less true over time, and that the early development of the language would be heavily influenced by the fact that most of its speakers will have their own native languages.