I'm not sure if you're proposing "inlikely" as an antonym for "likely", but that wouldn't be the same way that flammable and inflammable are related. The "in-" in "inflammable" isn't a negating prefix. It's taken from the Latin in (in, on) + flamma (flame), so put together means "to set on fire".
Hence the famous Dr. Nick line:
"Inflammable means flammable? What a country!"