It's hard to imagine what could replace the N900, especially after the upgrade to KP50 - combined with CSSU (Testing).
Seriously, I've been running on KP50 for 9 days and I'm blown away by the difference. I highly recommend it!

Now, the only things that I don't like are:
1. Screen is too small (my eyes can't focus on pixels that small any longer, the N800 screen was the perfect pixel-density for me). The current fad with all the companies trying to Keep Up with the Joneses (Apple) is silly. A perfect example is the people who think the iPad3's screen is awesome, but think my 21" tube monitor at work which runs the exact same resolution is way too difficult to read!
2. The proximity sensor is infuriating here in South Florida. It's nothing more than an IR LED paired with an IR sensor (a phototransistor or photodiode), so it picks up sunlight (even on a train with dark windows) and thinks it's at your face or pocket. The only workaround is to open the keyboard - more than a little annoying when you're trying to answer the phone.
3. The keyboard is too flat. I'm slowly getting used to it after these years, but it doesn't hold a candle to a SideKick2's keyboard. (sorry )
4. The radio doesn't have 3G(+) frequencies on AT&T's US network (the only alternative to T-Mobile). I despise T-Mobile and I get almost no reception at all when I visit family in Michigan... AT&T is everywhere up there. They also have so many dead towers down here that I can't hold a conversation while riding the train to work.

But, things that would be required to drag me away from this wonderful piece of equipment:
1. Open platform that's easy to access and tweak. The fact that this thing runs a real Linux distribution, complete with X and all programming and scripting languages (especially Python!) available is flat out phenomenal to me. I love the fact that I can use it as I use my desktop computer. The only thing that comes close is WebOS, and we've all seen how well that went over... so I'm not holding my breath (though I am keeping my fingers crossed for Meego/Mer/Nemo/etc.).
2. GOOD keyboard. Something with depth as well as a realistic number of rows.
3. SDHC/SDXC card slot. 2 would be better... I still love that about my N800!
4. Bigger screen with less pixel-density. Give me the pixel-density of the N800 (not N900) in at least a 4.3" screen, and I'm thrilled.
5. Bluetooth + NFC.
6. IR and FM transmitters (I know, not likely). The fact that it has an FM transmitter completely blew the minds of several coworkers when we were on our way to lunch and I wanted to play a tune for them while riding in the back seat of my coworker's car. It's not about showing off though, it's about the amazing convenience and versatility it provides.
7. A stylus. Seriously... I find capacitive touchscreens to be handy for very casual use, but nothing more. They're painful if you need any accuracy whatsoever, and impossible if you need precision. Just try and play something like Numpty-Physics with your finger!
8. Good optics for the camera! I love the optics in this thing! The only improvement could be the control of them.
9. Brilliant communications integration. The way this thing uses one interface for all IM protocols is almost as great as the way it integrates VoIP into the phone-app!
10. Decent battery life.
11. An amazing community who not only provide intelligent support, but actual hardcore coders making massive improvements to the device. Of course the latter couldn't be possible without #1, above.

I could go on and on, but I'm extremely tired and have to get up for work early... so I guess my answer is:

It would take something on-par with an Adamantium Unicorn to make me want to switch to another device.