Learn English – Antonym of ‘Formalize’

1. You might get some insight from this...I am suggesting that "nonsensical" is an unpaired word like "unwittingly".
2. Possible antonyms:
   • sensible
   • coherent
   • logical
   • intelligible
   • meaningful

Sparse and its antonym dense may serve in some contexts, such as mathematics and computer science. For example, a sparse set is a set that has few members out of a large potential population of members. An empty set has no members, and a dense set has many members. However, I can't recall hearing the term full set in this context.

In other contexts, such as "a glass half-full" one would not use sparse or dense.