They can't (or won't) do more spectator slots (I've brought it up with them a lot. xD ). Like ramjam pointed out, there was issues with massive lag just from the sheer number of spectators. It was cranked down to 4, and I don't believe they have an intention of changing that.
I think having a better way of finding the streams for the matches would be better. It's hard to find a competitive match that's not streamed. Usually you'll get cepsorts or therobanddan show with something, but when they're tied up, there's a number of other streamers that tend to sub in. I've done this on occasion (although it's not comparable to an actual caster).
Available casters aside, one problem with streaming is that once you start, you can't advertise your stream anymore. You're in the match and have no way to interact with global. Sometimes other players fill in for them and post the link and say what's going on, but only sometimes. It'd be better if the stream was integrated into the game or they were able to make it more visible that they're streaming a certain match (Such as a little icon next to the match you can click to link you to the twitch, or a stream tab in social, or whatever).