Limit the number of streams/chats that a single account can be logged into simultaneously. I've noticed that there are a lot of bot-accounts that appear in EVERYONE's chat rooms, but never as viewers. These accounts seem highly suspicious, and I personally would feel better if they were no longer able to "infect" twitch with their frivolous logging of every chat's activities. Besides the fact that the chat replies from Twitch could be used as cheap DDoS source, they could also be exploiting the chats to infect their users in some way.
I think that a limit of 10 chats/streams for a normal user would be way more than enough for any one person. And for any known bots that need to be in multiple chats at once; an option to allow them could be put into the channel settings for the streamer to permit them on a per bot basis.