Nintendo have to make any new unit vastly different from a typical phone (and different to Vita) to survive. That means leaving 3D in and sticking with dual screens. I have no doubt the 3DS will be sold for at leat the next 3 years with at least one other revision coming.
what would make me buy
* sunlight-viewable screens
* voice (walkie talkie) between units along with the typical drawing collaboration between units (I've always wanted voice chat)
* an actual effective offline travel clock / organizer / calendar / contacts/ alarm app without using an NSA spyware-infested phone. I think the market for this is huge with kids especially. Especially if Nintendo made it unique and more fun.
* cameras that move beyond VGA
* extra-rounded shoulder buttons (similar to 2DS)
* removable battery without using tools
* removable MicroSD without using tools
* USB connectivity (with USB charging cable supplied). This is so long overdue. They can always sell a separate AC adapter for faster charges.
* a nice jump up in performance & resolution (an actual 16:9 screen would be nice too)
branding
* they need to change the branding. It's confusing enough as it is now. "Nintendo Go" or similar branding would be cool
backward compatibility
I'd say this is guaranteed and so I don't think anything radical will be done in a new portable. Nintendo will not take the technical lead so they need something that their phone & Vita competitors don't have. Backward compatibility is a mjaor weapon they'd be crazy not to use. The only reason for not using it would be making something radically new (and maybe offering downloadable older titles on their store rather than DS or 3DS cartridges as some sort of concession). But I guess answers like this depend on lots of things like developer pressure to increase specs, ease-of-development, hardware costs, etc. I have no idea about this world. Part of me wishes for a single-screened console from Nintendo though. Or some sort of radical jump in performance & specs with 2 screens. There is a risk that all the shiny new phones with gorgeous screens (& so on) are changing perceptions of the DS line-up negatively too much.