Pho is usually the whole meal itself. You have the bean sprouts, chiles, scallion, herbs and such to add in to the pho at the table. It's complete in itself.
If you want to break with its normal serving method then I'm a fan of sliced cucumbers in nuac cham. Don't make this ahead as the cucumbers will leak too much liquid into the dressing.
Or maybe just some sliced fruit. Star fruit, mango, maybe dragon fruit if you can get it in season.