Nintendo could do quite a bit, but I'm going to mention what I think is likely.
Let's see... a $20 to $30 price cut and/or a redesign is likely IMO. By the end of 2014, it will have been over 2 years since the XL, and close to four years since the 3DS launch. Thus, a 3DSi might be released with updated features and functionality. I'm guessing a better battery, superior build quality, better wireless capabilities, etc.
As for games, it depends on what's coming out in 2013. I suspect we'll have Kirby and Zelda games later this year, which will make up Nintendo's holiday lineup along with Pokemon X/Y. If those games don't come out in 2013 for some reason, they will almost certainly come out sometime in 2014.
Super Mario 3D Land 2 or another 3D platformer is a possibility. It helps that the original 3D Land left so much room for expansion and development. Maybe EAD Tokyo will make a proper hub world ala 64, or a map to explore like in Super Mario World, or maybe even a multiplayer mode.
Besides that though, I'm predicting 2014 will be relatively quiet on the Mario front. The 3DS already has a 2D Mario and a Mario Kart, will have two different Mario RPG's, and a couple of other sports games by the end of 2013. That's all the basic Mario bases, and a lot of Mario games don't repeat within a system's life. The only major non-3D Land 2 Mario game I can see coming in 2014 is maybe a Mario Party, if only because the DS version sold very well (over 8 million!)
Let's see what else Nintendo has up their sleeves... the 3DS is overdue for a WarioWare game from SPD. 1-Up Studio might have a game done by late 2014, possibly an RPG, though the Mario-related name suggests otherwise. Intelligent Systems has no known upcoming 3DS game, so they're probably working on a new game for the system. I'm guessing either another Fire Emblem, Advance Wars 5, or that WarioWare title I mentioned earlier.
Pokemon remakes/Z are possible, but unlikely IMO; we'll see Z in early 2015 at earliest and Gen 3 remakes sometime in 2016.
So, in short, 2014 will likely be a letdown compared to 2013 in terms of 1st party support. But what about 3rd party games?
A Resident Evil game is not unlikely. Revelations might get a sequel, RE4 could be ported over, or Capcom could simply do something new. Sega is pretty likely to release a Sonic game for the 3DS in 2013 IMO, but it could come in 2014. Square Enix is overdue for a Final Fantasy game on the 3DS, either a remake or 5/6, a part of an existing series, or another new spinoff.
And then, of course, a lot of games released in Japan in 2013, like Monster Hunter 4, might take a year to come west, so there'll be those to play too.
Still, I once again expect 2014 to be an off year for the 3DS, if only because 2013 looks so strong.
tl;dr
Super Mario 3D Land 2
Mario Party 3D
WarioWare
Advance Wars
Resident Evil
Final Fantasy
Japanese imports
P.S. Forgot about Smash Bros! Yeah, that is another likely title. Maybe the star of Holiday 2014?
