Professional ways of saying "Keep me in the loop in case you have some other projects I may be able to contribute to"

I really enjoy working with you. I hope you'll think of me if an opportunity arises where I might contribute to one of your projects.

"Keep me in the loop" sounds fine to me. It's a pretty standard expression nowadays, and academia typically isn't hyper-formal anyway. I would just add "please".

I really loved working with you. It has been very productive for me. I would love to be able to do it again. Thank you.

You are this way letting him/her know how much you liked working together, you valued it (it was productive) and you would love to do it again. You are not asking for anything. You are not saying "keep me in the loop" that puts the burden on him/her, or anything like that ("let me know ...", "think of me ...", etc.)

You are making your point clear, no need to say anything else, and no pressure whatsoever on him/her.