“Fast, Cheap, Good : pick any two”.
I am often reminded of this when considering constructed languages. Unfortunately the wishlist is not that simple!
One thing that might be desired is for the new language to be easy to use and quick to learn. This suggests that the language should have a logical and consistent construction. Verbs should all be regular and there should be very few “exceptions to the rule”.
Easy to use implies that the reader/ speaker is not required to have a particularly in-depth knowledge of grammatical theory. Esperanto and several other constructed languages have distinct endings for adjectives, adverbs and/or other word types. It is quite possible to be fluent in a natural language without being particularly conscious of such distinctions. I managed several decades of speaking English without even knowing the distinction between transitive and intransitive verbs.
Familiarity might seem an attractive feature in making a conlang user friendly. Many conlangs have drawn on existing natural languages for their vocabulary. Typically the romance languages are used. Examples include Novial and Lingua Franca Nova. An English based example is Inlis. Some of these conlangs have the advantage that it is often relatively easy to work out the gist of the meaning. Easy, that is, if you have some grounding in the languages on which the conlang is based. Knowledge of the romance languages is not as widespread as one might assume from the number of speakers. I once overhead a long drawn out conversation between a French speaker with limited English and an English speaking Chinese girl. The stumbling block was the question “Quel age as-tu?” Another stumbling block is that the same or similar words have different meanings in some natural languages. English has so many homophones and irregular verbs that attempting to make it into something more consistent and logical rapidly transforms it to something that is not that recognizable.
Using existing words and words that are logical or easily learnt meshes somewhat uneasily. The word for plumber in many European languages is based on a word for lead, but it is not necessarily based on the word “plumbum”. Plumbers seldom use lead for modern plumbing so the name is not obvious for those that do not know its historical origins. “Water pipe worker” seems more obvious but that could also describe a urologist! We could replace the word for “worker” with “doctor” or “healer”, but a urologist may be an academic rather than a medical man. “Water pipe studier” could mean someone who designs drainage systems!
Brevity is another desirable characteristic. Obviously it would be desirable if the most commonly used words are the shortest and/or easiest to say. This may clash with using existing or familiar words. It may also clash with more logical construction of words. Even if we assign only a syllable to “water pipe worker” we have a word of three syllables, four if “worker” is formed as an agent noun of “work”.