Well Im a site designer, and frequently utilize phplist for clients. What I would llike to see is an ability to make it more like majordomo so my not so pc literate clients can just send an email to a particular email address and have it go out to all subscribers of a certain list.....but since this isnt an option.......
Dynamic retrieval of contents of emails ([URL:http://somesite])

Sorta like a replaceable blog? LOVE this idea!
More flexible frontend design
Forgot password system for encrypted passwords
Click through tracking
Send Emails in batches (eg 10 every hour)

I dont have huge lists so this doesnt bother me so much
Statistics/Graphs or subscriptions, emails etc
Blacklisting unsubscribed users, so they can't be re-subscribed
Date attribute for users that can be used to invoke an email
Multi Language support