Personally if I were in charge, I wouldn't change the Sidekick at all with regards to making it more like a traditional PDA. It may be geared towards kids and young adults, but it really has that market cornered well. There is not a phone brand more well known to that market with any company and there are lots of people willing to spend lots of money on the newest Sidekick every time it comes out. The inability to install your own apps brings in a lot of money for T-Mobile. I see the bills of people with Sidekicks and they spend a lot of money buying ringtones, games, and customer Sidekick Apps. A very large amount of people pay us $2.99 every month to have the MySpace app on their Sidekick. I say leave it pretty much like it is and start coming out with a few nicer PDA phones (like expected in the fall) to also grab that market.
With the topic on hand, I think 3G is a necessity on the next Sidekick after the 2008. I would be surprised if they don't add that. I really wouldn't expect WiFi or UMA though. It's not really a feature that would fit with the usuall Sidekick crowd. Lots of them would probably not use it or really miss it.