It helped me a lot to play around with different styles, instruments, softwares or even arbitrary limitations. It'll be a step backwards at first, but forces you to discover new things.
I agree with getting some drums, but more VSTs hasn't helped me personally - it's mostly just more clutter, more things that do the same things your stock plugins can do if you learn how to use them. SPAN, OTT, TDR Nova, Youlean Loudness Meter and Vital (all free) are the few exceptions to my "rule".
There's a free book I've been reading that I think addresses your question of "how to have fun again" - similar to what the first comment in this thread said.