Sideboards would be tricky to get right. I actually think it would cause a more narrow meta though, since you can adjust the best deck to be even better against everything. It might be interesting though.
I think sideboards are more important when you're playing the same person multiple times and since ladder doesn't have this, it's not important to have sideboards. Since your opponent is always random, your sideboard is changing your deck between opponents. It would be too good if you can see your opponents class and adjust your deck. If you can't see your opponent's class then you're already playing ladder as it is.
I'm not saying it can't be done well, I just can't see how myself and I don't think the upside would be all that great outside of a tournament setting where you play the same person several rounds.
good analysis that I did not consider