1)First off you should broaden your vocabulary when i was a kid my parents made me learn 5 words a week, this really help me with freestyling, the more words you know, the more options you have, also think of better ways, you can say basic words/phrases
Ex1 My lyrics fuck em up like a dick in a pussy
here's a more complex way"my verse hit whores like vaginal intercourse"
Ex2: i spit sick shit all my lyrics are hot
a more complex way" this thesis is apart of the chronicle of a genius, i got masterpieces that defy scientific reasons"
see small changes like this can make the difference between a basic freestyle and a complex freestyle
2) second you want the verse to have structure, start with a thesis, then every bar provides support for your thesis. this keeps you on topic and help you deliver a solid verse.
3)if you have a point were you brain goes blank try saying "Ok ,yea or a long wow" this give you an extra 2 second or more to think of something without falling off beat,