Since I tend to favor non-naturalistic languages like Toki Pona, Ithkuil, Lojban, and Solresol, I think of the concept.
What will my language do that no other does? Or, how can I extend and improve upon the previous attempts of other languages? Or, what constraints would be interesting? (Anglish uses only Germanic words while Tom Breton's AllNoun uses mostly nouns.)
If one desires to eschew high concept considerations, then I would advise to make a language as close to your own mindset as possible. Make no consideration of what is linguistically "plausible"; make it personally authentic. Bare your soul on paper. If it is good to you, that is all that matters. Don't try to base your language's worth on other people's opinions.