Is that an Mbox 2 or Mbox 2 Pro? Sure it's an older interface and you can do better today but I still want to make sure it really does not work. e.g. here is a report of the Mbox 2 Pro working on High Sierra:
http://duc.avid.com/showthread.php?t=395943
Edit: Doh I now see you said Mbox 2 USB. So I suspect you are out of luck there, but make sure you are not just blocked in the driver install security permissions.
What is your budget for a new interface?
Any idea what you mean by step up? Are you looking for more inputs, features like ADAT? Better preamps? Better monitoring capabilities? etc.
Examples of interfaces I'd look at that I view as significant step ups from the 2i2 are the UAD Apollo Twin (esp. if you want their console features) or Apogee Digital Quartet or some of the Motu products. Look at these to give yourself an idea of capabilities in fancier interfaces. One thing those brands deliver is pretty good support/software upgrades.
It's easy to focus on one item and decide to spend money on that, but the whole picture is important. Spending money on better monitors or room treatment or microphones may give you a better improvement.