You could look to pannier/backpack hybrids, these may be appropriate for your use-case. Personally, I solve this issue by having two different bags - a pannier for cycling and a backpack for other times.
Panniers can come in pretty large sizes, I'm usually able to accommodate a laptop + accessories + a change of clothes. Pannier racks fit two of them, so you can always double the carrying capacity when it's warranted.