Idt this place is super activate, you will probably have to wait
a little bit for a reply. I have a book called 'vocabulary builder'
which you can find with dictionaries. If you hear a word that you
haven't heard before look it up. That will for sure help you
with expanding your vocabulary. Also as you may already know
try working with multi-syllable rhymes. I don't really think there
are ways to improve your metaphors, and lyricism expect
keep practicing, and keep rap in your head. What I mean is
when i go throughout my day I will randomly think of a couple
good lines because someone mentioned something or i did
myself. If you only think about rap when you sit down and
try to write then you may miss out on ideas you might have
received during the day. You probably already know this though.
The last thing i can think of is just in general be smart, read
books ect, don't be afraid to gain new knowledge about something
you might not really care about or don't know about, and
research if you have doubts. Thats all i can really think of
hope i helped a little bit.