I'd suggest going back in time preferably to koryu kenjutsu, over Iaido to move forward.
koryu kenjutsu, iaido, whatever you call it - what really matters is what you want to learn and who you are learning it from actually knows what you want to learn. We workout doing kumi-dachi of some age so there is elements of koryu kejutsu in what we do. If I knew anyone of real skill within reach I would certainly check them out. It would have to present something more than just learning new forms. Actual principles.
I don't look at Iaido as something to move forward Aikido. Iaido exists in it's own right.
Aikido is broad enough and already has what these other arts offer, but extracurricular study is in my mind just like learning another language can give you a new look and insight to your native language and sometimes shine light on aspects of what you think you know but really don't know. There is also the aspect that there never is 100% transmission of anything from teacher to student so you always need to go out and experience and learn from anyone that has something good to offer. It makes it possible to find what you are missing (It's not about what the art is missing, I don't think the art is missing anything, but I could be missing something.)
Same for Iaido, koryu kenjutsu would be more in my Iaido but It's a matter of time and personal resources.