Worstgen
Cinera
For Luffy, a demonstration of advanced COA (both forms) and the stamina and base combat ability gains he made in Udon.
Cinera
For Zoro a demonstration of Enma, his improved Colour of Armaments haki prowess, and probably his improved offensive power due to the aforementioned two.
Cinera
I think Luffy fighting Queen or Jack (at the expense of Sanji, Jinbe, Dukes) and Zoro fighting Who's Who is plausible for the aforementioned reasons.
Cinera
Oda cares more about those two as fighters than anyone else in the Strawhat Pirates, and a lot more than the mid/week trio.
Cinera
If the question is: "would Oda rather have Who's Who provide a main fight fight for Brooke or serve as a hype tool for current Zoro?", I'm not sure why we should think Brook's fight would be more important than being Zoro's hype tool?
Garp the Fist
Why do they need to showcase the results of their training against characters we already know they are stronger than? What's the point? Zoro vs King is a struggle, Zoro vs Who's Who is a by-gone conclusion. Luffy's already beaten two Yonko commanders and been heralded an Emperor. I've seen already seen Sanji in a RS batter Page One across a city, I don't need a finishing blow to see how it ends.
Cinera
I'm not sure Oda will have them debut the results of their training before their final battles. I was merely providing what I considered a plausible reason for the M3 to get fights at the expense of the weak and mid trio.