Interested in your point about events that need a 16,500 seater host and that take place several times every week. (No tone and not intended in an argumentative way at all) - what type of events? Having considered the list below I could see that there could be a market for comedy or music that attracts more than 5,000 for the Motorpoint (but not enough to fill the Principality or City Stadium) but then that's not multiple times a week...
I'm struggling to think what could make use of a facility so often - especially when the current offered spaces cover:
Cardiff Motorpoint - 5,000 people
Cardiff Arms Park - 12,125
Cardiff City Stadium - 33,000
Principality - 74,500
Wales Millenium Centre - 2,500
New Theatre - 1,200
St Davids Hall - 2,000
IAW - 3,100
Sophia Gardens - 15,000