Playing > Project Announcements or one of the various competitions. Here, itch is mostly used for submitting to itch-based jams, not for hosting works. We generally like to have downloadable (and therefore archivable) files.
Not really. IFDB is the main place for that, and likely will remain the main place for that. See Storyfall for an example platform that tried to fill too many already-filled niches at once.
Different people prefer different things. As I’ve said before: a good platform has a low skill floor and a high skill ceiling. The parser-choice debate is probably one you need to decide on (or not—see Dialog) but it should be powerful enough for the author to decide for themselves what they want. Multiplayer is largely unnecessary, although a good multiplayer authoring tool would be something new.
I tend to keep the main idea simple and have deep levels of implementation. I just keep going deeper until I’m happy with things. So the level of complexity in my games is the level I’m happy with.
Also see: new-platform