As far as Nintendo goes, we know that they are focusing on games releasing in 2015 on Wii U and 3DS. NX and Mobile stuff are allegedly a no show this year. We also know Zelda Wii U will be a no show, and the majority of Nintendo's games announced last year are already out or on the verge of releasing, and they haven't announced anything new this year so far. They've also stated that they will have more games on both Wii U and 3DS this year. With that in mind, I expect:
-A brand new game will be announced at the Nintendo World Championships 2015 to get people hyped before their E3 events have even started proper. Super Mario Bros. 3 was originally revealed at the original NWC in 1990. It may even be a new Mario title again this time (Galaxy 3, to really stay true to original NWC, by announcing a 3rd installment in a critically acclaimed Mario sub series, as a 2016 game?).
-As far as already announced Wii U games go, I expect Star Fox, Xenoblade Chronicles X, Devil's Third, and Mario Maker to make big showings. Yoshi's Wooly World may get a small trailer too, but that game has gone gold already (it's literally coming out two weeks after E3 in Europe and Australia) so I don't expect to see much from it. We may also see more of Project GUARD and Project Giant Robot, but I heard the former might not actually be game in itself, but rather factoring into Star Fox in some fashion (this was also hinted at in the game itself last year, as the Star Fox logo was on the cameras or something). SMTxFE may show up too.
Also, Smash Bros DLC stuff is a given too.
-As far as Wii U games getting their first unveiling, I expect a new Paper Mario game, a project from Next Level Games, and Retro Studios, and maybe a new IP. I can't guess much beyond that, though. Hoping for some pleasant surprises.
-I admittedly am out of loop on 3DS games, but Fire Emblem: If could make a showing. I also kinda hope a 3DS exclusive Sonic game is announced (I don't expect a Wii U one, I expect the next big console Sonic game to be multiplat, and to not be showing at this E3 due to SEGA not having booth or a conference this year. However, they are still going to have games announced via their partners, so a new Sonic game on a Nintendo console isn't out of the question). The Great Ace Attorney may also appear in Nintendo's Digital Event and Treehouse Stream as a third party title.
-Indie reel is a given. I expect to see Freedom Planet there at least. No other expectations specifically, though.
-Amiibo news is practically a given. New Smash Wave could happen, or a new Super Mario Wave. They may even announce a new Amiibo line altogether for a new game. I'd also love an action game that uses all previous Amiibos to load up characters, or at least a majority of them. This would be great because Amiibo users would have a new, Amiibo-centric game to look forward to, but don't have to buy a bunch of new ones, they can just stick with the ones they have.
-At every Nintendo announcement event, I will always hope for a Rosalina game on Wii U. Absolutely needs to happen someday. It's not a specific expectation for this E3 though, it's just a perpetual hope in the back of my mind.