I would love to see a way to conceal spoilers in the chat. That way mods don't have to struggle as hard to police them in blind streams, and chatters aren't as limited on what they talk about when in a stream with a blind streamer.
Spoiler Tagging would be one of the biggest upgrades to the overall Twitch experience for a lot of streamers, especially folks doing Let's Plays. Right now the only way to deal with viewers posting spoilers is to time them out or clear chat, which isn't a positive viewer experience. Letting viewers opt into mouseover spoiler tags and letting mods apply spoiler tags to existing messages from other viewers would be an invaluable tool, and it would let chats for games sensitive to spoilers be much more active. Right now, there is a "walking-on-eggshells" fear for a lot of games and channels that doesn't promote a healthy chat environment.
Example implementation would mirror the current whisper functionality:
/s "Spoiler Context" Message Text
To post their message to chat, blacked out, with visible overlaid text suggesting viewers must mouseover/click/tap to view the spoiler.
If viewers specify a "Spoiler Context" argument, this would also appear in the visible overlaid text. If they don't specify that argument (e.g. just type /s Message) a default context that just says "Spoilers" will display.
Mods should be able to mark existing messages as spoilers by clicking them and by using a separate chat command to mark recent messages from a viewer as spoilers.