Gear Stacking is when he combines Gear Second with Gear Third(bone balloon)
Pretimeskip, due to gear second's drawbacks in Enies Lobby(taking a toll on his stamina and reducing lifespan in continues usage), combined with gear 3rd's after-effect(turning chibi-mode), using both completely knocked him out against Moria, after 2 usage of the jet shell
But it's not the case anymore due to much greater mastery over his gears like seen against Yamato where he combined both gear2 and gear3 without any issues
And to be honest the current situation is pretty similar to thriller bark. He still has gear4 after effects(10 minutes timelimit due to the way haki works), so because of this i don't see how Oda could really explain giving Luffy a new mode on top of that.
However what i can see happening is Oda using a similar concept to the gear stacking he displayed against Moria, by making Luffy stack his gears at a decisive moment against Kaido
With that said, although he joked around it, the fact that Oda decided to acknowledge the potential idea behind gear 5 might mean that in a corner of his mind, he might try to introduce it
Gear 4 was the gatekeeper for commander level opponents anyway which have been the core development for the major part of post-timeskip, so now that it's heading for the next level(emperors), it makes sense for Oda to go a step higher as well