One thought process you can take when thinking of "what to do"...is to think about the company / games / styles you want to work for / on.
Research them and make effects that would fit their theme / style.
Want to work at Blizz doing VFXs...what do their VFXs usually look like, try to match them.
Want Infinity Ward? Recreate Call of Duty VFXs.
Or in the end for practice, either do whatever you want...or if you are starting out, start with the basics.
By basics I mean...fire, smoke, sparks, etc. Then try doing those basics in different style. Cartoony / Stylized fire VS realistic fire...and so on.