Add unified mailboxes for folders like Inbox to display content of all inboxes from all accounts in a single folder.
By default create unified mailboxes for Inbox, Sent and Drafts folders and allow users to configure the source folders for them as well as creating custom unified mailboxes.
Implementation-wise this is done as an Agent that creates virtual collections representing the unified mailboxes and watches all source collections for changes and links any new Items into the respective "unified" collection.
The almost-finished implementation is in dev/unified-mailboxes branches in kmail and mailcommon repos.