1. Don't bet on an update. Samsung barely adds or back ports camera features. They rather you upgrade. Last time they did that was for LLM on the S3, and it still didn't get the Note 2's Slow Motion. It wasn't a processor issue cause my HTC Vivid with a Snapdragon S3 SoC and no ImageChip could do 720p slow motion. They just didn't back port it.

2. Slow Motion on the Note 4 is practically the same as on the Note 3, so that really isn't a reason to upgrade. The 3 also has 1080p at 60 FPS. OIS is the only major selling point from a video perspective. The colors in video are even a bit dull and the picture a bit more blown out compared to the 3, if reviews are any indicator (but it has OIS).

3. Yes, it will go unchallenged until the Note 5 (at least), however 4K recording on the Notes is likely to go unchallenged until the next iPhone at the very least (though that's less useful than what you're asking for, admittedly).

Personally, I'd rather Samsung work on their Photo Editor (extreme resizing of photos you edit) and Video Editor (missing tons of features) than add more fps. 120 is really good already, and without the software to really work with it adding more is not of great benefit (unless you love running to your PC to edit, etc.)

4. The colors in video on the Note 4 already seem a bit more muted than the Note 3, so it may suit your fancy out of the box already! And it has OIS to stabilize without cropping the frame. It also has 2x lossless digital zoom which works great with the OIS.

5. I don't expect Samsung to back port much, but it's possible and part of the reason I'd never update for software features when we know a major FW update will be incoming (Android L).

6. Not something 3rd party devi can do because this platform doesn't have the necessary frameworks and APIs to facilitate it. iOS has for a while, which is why they have superior 3rd party camera apps. Android L is supposed to remedy a lot of this, at least for pictures, soon.