I have to say I'd really love the ability to see multiple families.
I'd also like to be able to copy and paste individuals/families/criteria match individuals from one tree into another - both being visible. I appreciate the Export facility is there, but ...
Also, I fully concur with the post on data integrity - I have bizarre situations in my main family tree (with now 90k people) that I just seem not to be able to fix. After over 10 years of gedcom software tooling I am coming to the conclusion I need to start again on this, from scratch and rebuild it one person at a time. It would sort out the thousands of sources and places that just don't do it for me any more because of my files' successive merging history.
Regards, James Usmar