The most important part of a rap song is the chorus. Verse Verses in rap are usually 16 bars in length. It is all art, do it with your creative talent and style but make sure that the listener will be entertained.
One of the best ways to structure your ideas is in groups of 4 bars. Rhyme First Method Think up a bunch of cool rhyming words that are generally relevant to your topic and then form lines around those rhymes.
So nowadays, to sound current your rap verse should focus on both the flow and the melody. You can chop in vocals for creative effect and also when certain parts of the beat hits etc. Focus on the message and how you spit it.