Justin Mecham has a fantastic wishlist for Series 4 TiVo features he'd like to see. Most of them are season pass related and I have to say I agree with them all 100%. Some of them are a bit complex, but I could picture the menu options in most cases (like setting defaults for season pass options).
For myself, in addition to everything Justin mentioned (especially Hulu integration), I'd like to see the two-way CableCARD technologies implemented so that video on demand and pay-per-view content directly from cable companies could be accessed.
I'd also like to see other ways to be notified by my TiVo, perhaps by RSS, when an important event is coming up. I keep a bunch of wishlists around that I don't set to automatically record, but I often forget to periodically check them for the 1 in 25 upcoming items I do want to see. If I could subscribe to a feed of upcoming keyword/wishlist items with a handy link to record, I could easily scan recent matches and ignore or record items without having to dive into deep menus on my TiVo.
In addition to Hulu, I guess I'd like to see something more like boxee where I could play any media file on my TiVo (which is just a small computer capable of running anything), but given TiVo's ties to the major studios, I doubt that ever happens.