Microsoft: Promises a brand new, revolutionary lineup, then proceeds to talk about the following: At least Five Core Gamer games (three of which will be FPSes, one of which will be some low fantasy/semi historical if you squint really really hard game where you're some Marine type going around stabbing people in slightly shinier ways, and one modern day/semi futuristic Marine type going around ganking peeps), Two Racing games, One AssCreed game, One EA game, a bunch of rambling about the
360XBone's amazing multimedia features (featuring no less than two PR guys and one Celeb making fools of themselves), a bunch of Kinect Shit (with the requisite pair of kids trying to have a ball in spite of the script telling them how to have that ball), a Time Share-ish infomercial couched in the idea of shilling crap that has nothing to do with gaming to the audience, and one 2 minute Trailer of The Big New Hotness at the end. Basically, the exact same shit they done 5 years running.
EA: You've probably already seen everything they're going to offer (either through pre-E3 trailers, or from the Microsoft conference mentioned above). There will be exactly one announcement that they will build their conference towards, it will not be worth waiting for.
Ubisoft: Will probably have a hilarious (or hilariously awful) host. Will unveil the same Ass Creed as from the Microsoft conference (since the franchise pays their bills), will probably unveil more ports of Rayman and Scribblenauts, will probably announce some Watch Dogs DLC, they may talk about last year's post apocalyptic co-op survival RPG (provided it hasn't already come out), and they will end their show by unveiling something genuinely interesting or relevant to my interests (like a Child of Light 2 or something) and thus win E3 with a single master stroke.
Sony: Will not be nearly as entertaining as they were last year when Microsoft decided to dig the XBone into a hole and Sony promptly buried it for them. They will however, show off games with a generally broader level of appeal compared to the offerings mentioned above. They will however, also mention something really dumb like the Wand, the Storybook, or some other gimmicky piece of shit (unfortunately, this probably means The Occulus). They will also remind us on just how much they love Indie game devs. There will be two new games worth keeping an eye on (one of them probably from Squeenix). And finally, they will announce new bundles for their PS4/Vita to help make PS4s even harder to find/people give a fuck about the PSVita.
Nintendo: Will probably spend most of their time on Smash Bros, Zelda Musou, and the Xenomon remakes ("I am the Alpha Sapphire and the Omega Ruby! Ye shall be as Pokemon Masters, like no one ever was!") this year, and will remind us that the reason why we're still holding onto our Wii Us (i.e. X), is still coming and will still likely be awesome upon arrival (if the system isn't completely dead by then). They may necropost something about SMTxFE and announce some other crossover title like a Metroid Metroidvania or Pokemon Kagura (where Pokemon Ninjas battle and strip each other). They will also talk about that Skylander knock off shit they were musing about a month ago (that like every other Nintendo peripheral since The Zapper will crap out after two games) and other attempts at not ending the year in the Red due to all the money they're hemorrhaging over the Wii U, lest Iwata's out of a job.