Great idea.
I would envision it just like the account screen is today, but only with the possibility to group accounts together. The grouping could be given a name and the name would appear above the group of accounts.
Arranging accounts would be easier then ever because you could drag and drop the group as a whole.
And the best thing: you could collapse a group. This would make the account screen clearer and would also kind of implement the wish of a lot of bunqers to be able to hide certain accounts, just group them together and collapse the group.
When a group is collapsed is might show the aggregated amount within the account of that group.