I bet Nintendo will "win" due to (hopefully) Crash in Smash, which would help promote the N-Sane Trilogy coming out one month after E3. Tropical Freeze was ready to go in November 2013, so Retro's game should be revealed--hopefully a new IP. Cardboard Yoshi is likely to show up, but I'm not so sure about Fire Emblem since they keep releasing those games a year earlier in Japan than anywhere else. Then again, I suspect they'll announce Animal Crossing Switch even though that series has the same release pattern. Nothing more than a teaser for Prime 4--like 10-15 seconds of gloomy rainy planetary scenery and then BAM Ridley gets the drop on Samus! "2019 OR 2020 WHO KNOWS"
Sony will have another solid showing. Last of Us Part II gameplay (like Uncharted 4 at E3 2015), probably Ghost of Tsushima, maybe the mysterious new From Software title. Honestly I don't know how they'd show anything from that zombie horde game for a third E3 in a row that would get people excited; I suspect it's becoming Sony's take on Crackdown 3--a game without a too-familiar premise that was announced too early that too many people have bailed on.
Microsoft's my wild card. They almost had me last year with the "100 exclusive* games" nonsense, but it's all for naught if they suck or nobody cares about them. Now I did get Super Lucky's Tale at launch and ReCore the previous year, but nobody else cared about them. Microsoft really has the worst luck--they've got a bunch of new IPs this gen (including Ryse, Sunset Overdrive, Quantum Break) but none of them's moved the needle except for Sea of Thieves.
But what will they show? Forza Horizon 4's a lock. Some have their doubts about Halo 6, but I think it'll be revealed for release this holiday--most likely the story will suck again, but if by some miracle they've salvaged it, E3 is Microsoft's to lose. Keep in mind, it might not be the end of the arc since it's called the Reclaimer Saga now instead of Trilogy. Finally, back to Sea of Thieves--its first free update (The Hungering Deep) is coming in May, so they'll keep the momentum going with a trailer for the second expansion, Cursed Sails. And Crackdown will either blow our socks off with a re-reveal of a re-tooled game, or it'll be more of the same, coming out this time for sure!
Ubisoft, Bethesda, EA--I honestly don't care enough to predict their showings.