Few things I think need to be added to with the joint account:
Ability to request money from your personal account on either side of the account. For example if we go shopping and I decide to buy something for myself I can just request the amount from my account to avoid paying in two separate transactions or sending it proactively.
Ability to schedule a payment to the joint account without having to use the account number and sort code. This is only a minor annoyance
Ability to add money into the joint account pots so we have a joint holiday pot but at the minute we have to transfer the money into the joint account. Then then move it again within the joint account tab.