Steve saying it in "Avengers 4" seems unanimously a given, I think everybody's on board with that. I don't see how they go through "Avengers 4" without having it be said. I mean it just HAS to, right?
Now as far as context, I couldn't say. It feels like the right way to go is to motivate their last ditch assault against Thanos, since in "Infinity War" you see them attacking Thanos one by one in Wakanda...so at some point, perhaps 'Assemble' is the actual plan i.e. we're all attacking him at once from all sides and we're not going to let up until that Gauntlet's off or destroyed or whatever.
So maybe 'Avengers Assemble' becomes the mantra for future generations and rosters of the team after its use by the original members against Thanos, which becomes part of the team's lore going forward. That's just me speculating obviously, I have no idea why, when, where or how they'll use it.
Speaking of 'how' though...I think HOW Steve says it, or might say it, is more of a matter worth mentioning.
I, for one, would rather the line not be said in the loud, almost sing-song'y way, which you can see parodied here by Major Glory on "Dexter's Laboratory":
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It doesn't need to be some broad, over the top version where Steve is screaming it like that. It can be audibly loud so that everyone on the team can hear it, but I'm thinking and I hope that it's based more off of Steve as military, shouting an order from the trenches kind of thing. Very real, very sincere and honest, if that makes sense.
Does that make sense? Like if they make it this almost lyrical gesture where it goes "Avengers ASSEEEEEEEMBLE!"...like in the same vein as "Let's get ready to RUMBLE!"...that to me would be more fan service-y number one and number two, it would feel hokey.
The line being used is already fan service, kind of...so that to me means that HOW it's delivered is going to matter and I think a very direct, no-nonsense delivery is what would work.
