The fact the presentation is only 30mins long and focused on 2017 limits the number of games and new announcements likely to be made. After all, if they only spend five minutes per game, that's just six games. My best case scenario would be something like:
- Mario Odyssey big game focus (incl release date) 5-10mins
- Short piece on unannounced Splatoon 2 modes/features and free ARMS DLC ~5mins
- Short piece on BotW DLC packs (incl release dates) ~2mins
- Xenoblade 2 trailer ~3mins
- Mario vs Rabbids trailer ~5mins
- Showreel of upcoming 2017 games (Switch/3DS), maybe including a WiiU port a la Tropical Freeze DX ~3mins
- Features coming to Switch by the end of the year (Netflix, web browser, usual stuff) ~2mins
- Very short Metroid 2018 tease >30secs
I cannot possibly see how you could fit in more than that, and would be extremely surprised if they even did that much. Excluding perhaps a tiny teaser for a big 2018 title, I wouldn't expect more than two new games (Rabbids incl) to be announced.
I feel confident in saying there won't be that much time dedicated just to talking about Odyssey.
We know it will be in a playable state (as it will be on tour in select retailers during E3 week) and will also receive a substantial push in the Treehouse demos. As with their BotW strategy last year, I can see the presentation presence being limited to a single trailer with (a) one substantial feature we don't know about yet (like alternate playable characters or some form of co-op, if that can exist in a Mario game of this format) and (b) a release date, with the bulk of Odyssey content being shunted to the lead-off segment of Treehouse Live, the first half-hour following the Spotlight video.
Xenoblade won't get more than a trailer, and even then the thing to watch for is whether there is English VA and text ready to go in the trailer alone, the bellwether of a global release in 2017. Splatoon needs a feature push for a number of things that have been shown on video or Tumblr without direct explanation, like spectator features or local multiplayer, and it can afford to have some of the details shunted aside as it will get a whole tournament to itself.
I'm optimistic, in other words, that we won't see too much of a retread of the titles first announced in January. Odyssey may be the banner title but I expect the main push for it will be in Treehouse Live gameplay, while the Spotlight video will reserve as much time as possible for new announcements. The segment that people will probably be most impatient with, I foresee, is Rabbids, but that's mostly because the surprise has already been spoiled.
At minimum we'll hear about what the mass-market game for Spring 2018 will be. There is always something positioned like that. (That won't be Metroid; if we see Metroid, or Retro's game, it is likelier to be a tease for an unspecified 2018 release.) There is plenty of room for two or three first-party announcements for 2018 (I'd bet on two) even if the focus is on 2017 and Nintendo prefers short announcement-to-release cycles of half a year; lots of precedent for that in their strategy.
Honestly, I'd be more concerned about too much time allocated to what I call "blah blah third party blah blah"token shows of support from western publishers that are really only there for Nintendo to signal to the press that they have mass-market third-party support. I completely expect to feel my time is wasted with FIFA, NBA 2k18, Lego, Just Dance, and even more personally intriguing titles like Rabbids and the Skyrim port (a sure bet that everybody else seems to be forgetting) when I'd rather be hearing about the first-party pipeline, perennially exciting oddballs like Platinum, or at least some unique indie arrivals.
Three new first-party retail titles (late 2017, first half of 2018, undated 2018) are the benchmark for new announcements, though I'll concede that Pokkén/Rabbids might already have ticked the first slot. Smaller digital titles may sweeten the deal, though some may be set aside as an off-presentation Treehouse surprise (which I expect to be the fate of anything on 3DS, like the Kirby fighting game). That's the bar, and I think half an hour leaves just enough room to clear it.