Implement Terms of Service Article regarding Co-Authorship
Under German law (§8 UrhG), and certainly under the equivalent copyright laws of various other countries, anyone who participates in the creation of a work is entitled to co-authorship.
Theoretically, users who write in the Chat which is inserted in the stream footage can claim co-authorship of the stream.
Yes, in this case one can argue that as with a call on live television, it must be expected that one's own contribution (like the voice on television), will be broadcast live.
BUT: That streamers offer a VoD on Twitch is OPTIONAL. Also, that streamers record the stream and upload to YOutube, is not a given.
Thus, for the live stream it can be argued that common sense dictates that the participation is expected to be commercially used - but not for VoDs. This would mean that all commentators would be entitled to co-authorship as long as their comments are shown in the stream. And thus also a share in the revenues that streamers generate with them.
Likewise, it is a problem when, for example, authors write their books live on stream, incorporating ideas and suggestions from their chat (same goes to other artists implementing ideas prompted to them via chat). According to german, but certainly other countries' laws as well, the originators of the ideas also have the copyright and such after implementation of those ideas a co-authorship on the resulting work.
So my request is:
With a simple extension of your TOS, like
"By agreeing to the TOS, users unrevocably allow Creators, whose streams they comment on via the chat function, to commercially use the content of said chat posts royalty-free",
you would create legal security for your Creators.