For me there are still a lot I do not know about the R3, which I really hope the following info is released in the formal announcement or covered by reviewers of the camera:
1) What is the buffer depth and clearing rate?
2) Can the electronic shutter be used at speeds slower than 30 fps?
3) What is the flash sync speed with electronic shutter?
4) How good is the high ISO noise?
5) Are there cross-type AF sensors (I doubt it, but would be a nice surprise)?
6) Does the eye AF work well with normal eye glasses?
Hopefully, there are still some surprises!!
1. Your best source for this in a meaningful way is likely to be reviews. Initial impressions will likely be posted by people with test units immediately after the event. If it's mentioned in the promotions materials at the event, it will likely only be in subjective terms unless they're doing something very unusual with the system.
2. You also best looking at initial reviews for this. The kind of announcements we're seeing tomorrow tend to focus on the maximum, not all of the individual settings. The best you'd get would be *maybe* a breakdown of the drive modes, which might hint at an answer.
3. Expect something on the order of 1/200. Flash sync speeds with an electronic shutter are often limited by sensor readout speed. Sony has gotten flash sync to work with an electronic shutter at 1/200 on the a1. A lower resolution would give this body an advantage in this regard, but Canon tends to have a slower readout than contemporary Sony sensors. I can't see them even bothering if it's much slower, but it would imply some major things about sensor readout if it's much faster.
4. Expect subjective claims at the announcement. Early reviews will mention it, and mostly assess it as either "the best ever" or " a total embarrassment". A mix of the two extremes is a lot more likely than any sort of balanced assessment being common. Really though, we know what to expect. The differences between noise from one camera to the next, of the same format and technology, are minima. Most of the advantages touted in marketing materials come from improved noise reduction in in-camera jpegs, but even that's getting pretty small from generation to generation. BSI sensors tend to have a small, but noticeable, advantage in this regard, and this will be Canon's first ILC with a BSI sensor, so that's a bit of a question mark, but it's most likely a very small improvement over the 1DXIII, R5, and R6, most noticeable at very high ISOs.
5. Cross-type vs non-cross-type is only relevent to phase-detect AF systems. Being a mirrorless camera, calling the R3's AF points "cross-type" or "non-cross-type" is meaningless, unless Canon introduces something new, not directly related to PDAF cross-type sensors, but names it "cross-type". The closest corrolary in mirrorless systems to cross-type AF systems is probably larger AF points.
6. This will pop up in early reviews, but is unlikely to be directly mention at the event. You'll most likely have to specifically seek out reviewers who wear glasses. One note on a specific source- while Jared Polin wears glasses and has been very vocal about the eye control AF, he has some eye-twitch issues that made the previous generation (like on the EOS 3) not work for him. Even he's saying he might not be a good source for that, but he's been having somebody else on his team test that out.