The iMac's screen is only in landscape. The iMac doesn't have an on-screen keyboard. You can resize windows on the iMac (I never use pretty much any app full screen).
When typing in landscape if the iPad were 16:9 you'd have the keyboard fill almost the whole screen. In portrait the keyboard would be even more cramped.
And again, what would happen to the apps?
Apologies, I don't think I perhaps made myself clear.
I'm not saying I want anything taken "off" the height of the current design to make it 16:9 which is what I think you think I am saying.
I want "extra" added to either side of the current display.
In landscape (as most people admit to using) you would have "extra" pixels either side of the current screen (perhaps about 1.5" wider each side)