I would really love to have tunnels that you you can move Kerbals through and into other parts. Right now you need to put them on EVA and build the inside with plates and all that stuff. But it would be so cool to have tunnels! For example, I could have a science lab connected by a tunnel to a crew cabin. Then I could have Kerbals in the cabin and then move then through the tunnel to the lab as if it were one big part! This could be really fun for space stations and long-distance mother ships. Just being able to see Kerbals float around inside would be great on it's own.