It's not a super-big deal, but on the former Google Messages, the MMS sizes filled the screen, at least from left edge to right edge. Annnnnnd, there was a preview of the MMS sent/received in the conversations screen.
It would be really awesome to have options to adjust MMS onscreen size, and if you want MMS messages previewed in the conversation screen!

Apart from that, BEST MESSAGING APP PERIOD.