I would certainly like an ‘integrated’ multi-post system that would allow content to be posted to up-to 4 other communities, and it would wrangle all posts into the same comment section.
Slashdot-style voting is another one. Or even bland-voting, with some sort of tagging system (the same overrated, flamebait, informative, funny, etc) – The bland-vote allows no-thought up/down votes for spam control, etc – while the tag system lets you sort things you want to see like insightful for philosophical debate, informative for just factual information.
Honestly I’d love to see more tag systems just in general anyhow - they’re great for searches, they’re great for categorization.