I prepared a list of 100 prompts that you can use to find your illustration style. Some days I was pulling my hair out thinking “what should I draw today”.
I tried to compose a list of various styles, themes, objects, illustration uses, and things to think about to discover yourself. It could definitely give you a good variety for the portfolio before you find your secret sauce and start to specialize.
If you manage to commit to something for 100 days you should also build a habit and illustration should come very easy on a daily basis. If 100 days seems scary, feel free to use the list as you wish.