I'm not entirely certain I agree with this in its entirety... after all, a fixed hand size opens the path for a future card o the form "increase the maximum number of cards in your hand to X", which would vastly change the ideal makeup of most rainbow decks out there.
However, what DOES annoy me is the way if one does draw when one had 8 cards in hand, the drawn card floats out of existence - you never know what card you drew, and there is no way to recover it. Maybe either forcing discard if you use a draw ability with 8 cards in hand there and then (at least your draw finishes, and presumably you can always cancel the ability), or some other method to voluntarily discard cards... Or alternatively, as a quick fix, increase the maximum cards in hand from 8 to 10, but force multiple discards at the end of the turn if necessary to get back to 7. It's fairly rare that in 3 cards over the max you won't come across ONE that you want to play immediately...