No Luffy would not be a headliner, he would be made a calamity. This is directly referrenced by Kaido's men when Luffy brought to Udon Prison after fighting Kaido. They compared Luffy to a calamity.
Lol they didn't compare him to a calamity..
They said they thought luffy was as Big(size) as an Okanban,not that he will be made calamity Lol. So Kaido's fodders are now the ones to determine which one is to be made calamity?
For the position luffy would've taken in the beast pirates,we know the 3 calamities spots are locked and can't be increased since the Tobbi Rollo are actively trying to challenge the Okanban for one of the 3 spots instead of asking Kaido to increase the number of spots..
Queen asked luffy to join them in chapter 936 and we had King pressuring Queen to break the wills of Kid and luffy in chapter 925,we also had Kaido asking Jack to bring him law and luffy in chapter 922..
What do those 3 instances have in common? Both calamities are willing to let those guys join them,and since the 3 Okanban positions are fixed,the only spots that Kid and luffy could've gotten is between headliner and Tobbi Roppo.
The calamities will not be willing to accept guys that will take their spots since it has been shown there can only be 3 calamities at a time,so luffy and Kid will have been just in a drake situation where they will climb the ranks by showing their strength and given opportunities to challenge those above them.