Looking forward to working with you again someday! This phrase carries a warm, professional tone that suggests both appreciation for past collaboration and enthusiasm for future opportunities.
It subtly implies that the experience was positive enough to warrant repeating, while leaving the timing open-ended. The use of 'looking forward' adds a layer of anticipation, making it more engaging than a simple 'thank you'. I've found this works particularly well in creative industries where relationships often span multiple projects.