Let viewers vote on ads
Voting is integrated into Twitch chat. When there are ads to be played, let the viewers decide which one(s) should play. This weeds out ads which the viewers of a channel don't like or have seen too often, and it will give Twitch useful feedback on the longevity and/or interest level of ads to take back to advertisers.
To avoid spamming the chat with polls, the results of an ad poll will apply to any ads played on the channel for at least two hours.