i guess it's likely there will be a smaller switch or switch lite at some point but the switch itself is their next portable. I consider it more a successor to the 3ds than the wii u in fact I see it as nintendos way of abandoning the home console market without making it look obvious. I just sense switch is gonna be more popular as portable but time will tell...If the switch bombs (and I personally think it'll be a success) maybe then Nintendo will come out with a more obvious portable, but that's a parallel future that may never exist. Remember Nintendo hinted in the past their next console (ie what became the switch) would most likely be abandoning the need for Nintendo to make a home console and portable at the same time so they could focus on making software for a single machine at once and avoid (hopefully) software droughts.

I can barely fit my new 3DS XL in my pocket if my phone and wallet are already in them though and often just keep it in my backpack anyway. which I will likely do the same for my Switch. If anything though we are more likely to get a smaller Switch within a couple of years then a "true" 3DS successor. 

There won't be a new Nintendo handheld in the future, hybrid is the next big thing for Nintendo.

There likely won't be a "handheld" anymore from Nintendo. I think hybrid devices are their future, assuming Switch is a success. Beyond that, perhaps a mini Switch that launches without a dock or something could be considered what you're looking for. A sort of revision that is smaller and has better battery life, that comes out a year or two after March 2017.

Don't fit into your pocket either. Are they not "handhelds"?

Tablet form factor is the new form factor that kids prefer, that's basically all it boils down to. 

When I was a kid you sure as fuck did not put your Game Boy in your pocket, didn't stop me from taking it to school every day and on every long car ride/vacation.

I am curious though if you fit the Switch without the joycons in your pocket since it looks pretty small without them.

The original Gameboy, the original GBA, the original DS, DSi XL, 3DS XL, 2DS, PSP, Vita.

None of those can comfortably fit in my pocket and I'm a relatively big person at roughly 6 feet tall and 200 pounds. There is no way that any of those devices can easily fit in a child or average sized womans pocket.

I think alot of people overestimate the amount of people who carry around handheld gaming devices in their pocket. Most people play that at home qnd the people who regularly bring them out probably carry them around in their backpack or purse.

Can people stop calling Xbox One a successor to Xbox 360?

I could fit a 360 in my entertainment center but an XBO doesnt.

Yup this is what I do since sitting with my wallet, phone, and 3DS in pocket is not comfortable so I just carry it in my backpack. Will also carry my Switch in backpack so I can play in between classes.

Depends on whether the Switch is successful or not