I'm with the above that this thread brings nothing that's truly unusual. Not often in the box for many of them, but still usually there.

1. Seen and used this on occasion. Useful, if not the first thing that comes to mind.
2. Far less often than ^. For me, mainly for continuity trouble. I don't bind my roleplaying to a set story, and that's precisely the top function of a flash forwards unless it's a specific gimmick for the roleplay.
3. Seen this once, maybe twice on the guild alone since this registration and I know I've seen it before that. Not common, but definitely not that unusual.
4. I rather think planning out arcs - by most extensions, how the story ultimately goes - helps drop investment. It certainly does for me. If I wanted a set character arc, I would write a book.
5. I've seen this a lot more in beginner scenarios for 'developing' roleplay communities. Usually an elementary way to structure the roleplay, but also something I've seen at higher levels, including around here years ago.
6. Seen it. Basically the standard for tabletop/tabletop hybrids and features occasionally in freeform. Not my taste.
7. I'm in the 'no thank you' camp due to my stringent attitudes on character control. There's some opportunity, but it's not enough for me to use it. Also not uncommon.
8. Seen it in other communities and roleplay formulae (ie, certain video game roleplaying). In play by post it isn't too common, but it still rears its head on occasion.
9. I don't think this is used often enough, especially when a roleplay starts bogging down, the intent is a long term deal anyways and communication is solid.
10. If used poorly, this easily makes a mess of threads, and that's precisely what happens far more than not when this concept is naturally used. That said, I've also seen this quite a bit in more 'developing' communities. It's usually a mark of their unprofessional structure than a positive feature.
11. Good number of these (as described) all over various roleplaying circles.
12. Messy in groups, standard in 1x1 for the 'common' type, viable in small groups. In anything larger it takes the right chemistry. Not seen it much outside of standard 1x1 practice between 'equal' partners.
13. Seen this in PbP a few times and in nation roleplaying, but I do see where you're coming from. In 1x1 I have a few partners where this is basically the norm.
14. I don't really see much use by the first part, and I'd consider the second part something else entirely. Not really a new horizon for me outside of a gimmick.
15. Seen this in various degrees a few times on the guild and elsewhere. I try to do it myself more these days as a means of measuring progress and getting accomplishment out of a field that commonly shoots to an ambiguous goal and usually falls miles from that mark. Such checkpoints are an excellent opportunity for those involved to review and make sure they're "into it".
16. With the right group, sure. I recall forms of this in my 1x1 journeys, but nothing more than vague notes or a gimmick in the premise in groups.
17. Very underutilized. It might be interesting to see this referenced more, though there is a charm to the time ambiguity as long as the game flows otherwise.
18. See: Persistent World.[sub] Doomed, I'm afraid.[/sub]
19. This is a go-to style for various 'developing' communities I observe and the result is overwhelmingly a disaster. It can work, sometimes it can even be great (some of my best ended up being made on the fly) but more than usual one's expectations should not be high. Not uncommon in general, either.
20. This is why my compromise is taking things by scene, episode, or segment. Each 'chunk' can be a completed portion, able to survive as its own journey, achievable even if it takes months to reach. You can gauge health and interest by where the other parties are at when reaching it. This I believe is a more successful approach than starting from point A and hoping to eventually reach point B even if there are vague divisions of scene in the meantime.

My two cents; I'd throw in a dollar, but I'm broke, and being a floating mask pays poorly.