2016 would be reasonable for a new handheld and/or console. So, it'll be interesting if this unified architecture idea (which Iwata said would make both systems like brothers in January) will lead to their handheld and console releasing closer together or not. I mean when you think about a console life, 5 years used to be a decent number for a cycle and when you consider the Wii U's bad situation moving that ahead one year doesn't sound like a drastic move (but I think betting on it taking 5 may be safer). And most people have been saying 2016 for a new handheld for quite some time. My safe guess would be Q4 2016, handheld, Q4 2017, console -- but they could conceivably go Q1 2016, handheld, Q4 2016, console.
I've even had an idea of Nintendo releasing both at the same time and really advertising as brothers. Something like, you can by the Ultimate Nintendo Entertainment System Home Edition or the Ultimate Nintendo Entertainment System Portable Edition, but I think logistics would be an issue for that idea. (They would need to be going fully iOS style for this to work at all, though, because sharing a name would be a way to show games work between them, if you have to still buy games separate, and the plan is just porting then this doesn't work.)
It's really hard to support two separate devices, as we've seen. One always is dealing with a drought while the other is going strong. So, obviously Nintendo needs to go this way. Stuff like Luigi's Mansion on the Wii U would have helped during the drought.
It also can free them up from immediately having to work on making a game from a franchise for one system then working on putting a new entry on the other system. I mean think about Animal Crossing -- it works better on handhelds but they don't want to skip putting one of their popular franchises on their console. By using a unified architecture they could just worry about making AC and putting it on both, as opposed to having to finish on one entry and then immediately start working on the next one for their console.