Adding another flight ex MAN say at 11:00 or 17:00 or 23:00 would not offer anything realistic. Before someone barks at me about the EK departure times from LHR to DXB... that route is dramatically different and carries a much larger point to point amount of customers that rely on frequency between Dubai and London and not on connections.
Adding a flight to MAN using a similar timing to the EK29 at LHR could relieve pressure on the EK18, and also catch the back end of the main hub wave. Also, a B777 will add the cargo capacity that MAN is now well down on.
MAN however is much heavier with onward connections and the point to point traffic is minimal.
Actually, EK themselves have said about 28% of MAN passenger are terminating at Dubai, which, incidently may seem a small amount, but, it was the highest percentage of total pax for them. When you consider how many connection oppertunites are oresent on MAN-DXB-XXX, to see 28% are terminating at DXB is quite a healty number. That of course doesnt include and MAN-XXX-DXB pax on other airlines.
Its also worth rememberig that Dubai is working hard in becoming a destination in its own right.
In other words, its not as easy to rule out a 4th daily flight at MAN as you seem to think.