How would you go about defining such a language? What considerations would be important? How could you ensure anyone, from any culture, could learn to read, speak, and write the language?
So to clarify, are you asking how we'd go about designing a language that's optimized for universality? Or some other metric?
Or, conversely, maybe you want to create a corporation where all employees must communicate in a custom language.
Also worth considering is making the language as "familiar" as possible to a new learner. You could decide what phonemes your language uses by looking at the most frequently used phonemes in the most popular languages. You might decide to go a level higher and base your language's word for universal concepts (food, day, sleep) based on roots that are common across many languages.
Another way to make a language easy is to reduce the number of words a person needs to know in order to use it in the real world. I'm personally a big fan of Esperanto's word-building system, which uses around 30 affixes (in addition to the suffixes that define each word's part of speech) that can be attached to a root word to modify or enhance its meaning. There's not a limit to how many affixes you can have, the word just has to make sense. Every time you learn a new root word, you get a huge number of new words for free by knowing affixes. A language that has such a system can get by with a reduced number of root words. For example: "laboro" is the word for the word "work" or "job" in Esperanto. By adding only one affix you can arrive at words like "labor-ej-o" (workplace), "labor-ist-o" (a laborer by trade), "labor-ant-o" (a person performing labor), and "labor-ind-o" (a job worth doing).
However, it's not really enough to just make a language easy. I'm glad you brought up virality; drawing people in is almost certainly the more difficult part. Once you have your ideal language, you need to build a culture around it. This is the same thing as when people judge the quality/utility of a programming language by its "ecosystem". Unfortunately this is largely out of the language creator's control. There's just too much work involved for a single person, or even a small group of people. There needs to be an abundance of learning material, lots of translated works and a growing body of original works (across as many genres/media/levels of proficiency as possible), and networks for speakers/learners to communicate and collaborate.
Thing is, a live language always comes as an organic part of some culture. In other words, a language is only live if it is absorbed together with mother's milk. For instance, I imagine it took a few generations for Modern Hebrew to really come to life. Chances that an artificial language could become part of a culture are slim.
From this perspective it's a very easy language to read/write (negating kanji) and pronounce.
Currently I am learning thai where each character have two sounds depending on where in a word it is located. Multiple characters with the same sounds, a few characters which are marked as obsolete (but still in the alphabet) and each word will have a tone based on the groups of characters (which has a "class" assigned to them) and a whole bunch of other "special" characters that change the tone, completely silences a character or repeats something. Seemingly illogical pronunciations when combining other sounding ones (combining a vowel sounding "e" and "aa" will pronounce an "ao" sound).
And I haven't even started on anything but learning how to deal with the alphabet.