On top of being able to have exclusion rules to the workflow, an admin should also be able to manually change an approver from the Approvals tab directly.
Let's say a particular invoice went to X for approval, but X is on holiday, I don't want to change the whole workflow, I just want to re-assign that invoice manually to Y as a one-off, without affecting any other items awaiting approval. A delegate rule could work, but there should be an option to do it as a one-off too.
At the same time, I need to be able to modify one workflow and see that modification applied to all items awaiting approval affected by that change, without these being sent back to Inbox - the approver would then be automatically informed. I also noticed that if now I change one workflow, at the moment ALL items awaiting approval are pushed back to Inbox, not just the ones affected by that particular workflow. This creates an immense confusion and duplication of work, as I have to go back and re-check every single invoice and re-publish it.
These changes should also appear as automated notes at the bottom of the invoice page, with the ability to leave a comment, so that the approver can see why the workflow was changed (for audit trail purposes).
Finally, approvers should not be able to see other's people invoices, but only the ones going to them for approval - for confidentiality purposes (except for Admins).
And no approver should have the power to amend coding/descriptions - they should only review and approve and suggest changes, which are then checked by finance and actioned if appropriate.
Any invoices awaiting approval should also be already visible in Xero as Awaiting Approval, while at the moment they don't appear there until actually approved.

there should be a feature to transfer approvals to another colleague. sometimes our engineers work in different sites, managed by different project managers. so we should be able to ask or transfer approval of a Expense Claim easily without going through admin.