Okay, so here’s a screencap I wanted to bring to your attention, and I haven’t seen anyone talking about it yet, so if someone else already thought of this then I apologize.
But… this is from the season three trailer, and Allura says the line “It’s time to think about finding a new pilot for the black lion.” Lance was in the previous scene, but he’s not here. The way I see it, there are very few reasons that Lance wouldn’t be there with them. Maybe the actual one will be completely different, and I’m just overthinking this, but oh well.
One potential is that Lance left the room himself, which is entirely possible. Maybe he was too upset to think about replacing Shiro, or maybe he angrily stormed out after Keith mentions Shiro wanting him to be the black paladin. He also could’ve voluntarily not gone, because he didn’t think the black lion would ever choose him.
Another option is that Lance wasn’t asked to go in with them. Maybe everyone else didn’t think he would need to be there, as the black lion would never choose him. This seems a bit unlikely in my opinion, but it’s an idea.
Or, what if Lance isn’t even in the castle at this time? Some parts of the trailer make it seem an awful lot like something bad may happen to Lance, and I personally support this theory from @inkedmackenzie. For all we know, he’s already missing at this portion of the story.
The last possibility is pretty unlikely, as we saw Keith in the black lion in the other trailer, but… I still wanted to mention it. Maybe Lance isn’t with them because he’s already inside the black lion. Maybe Keith leading didn’t work, so Lance figured he gave it a try. Later on in the trailer, we see the black lion turn on. But we don’t get to see who’s in front of it at that part. Why wouldn’t they show us, since we already know from the other trailers that Keith is going to pilot it? Unless… it’s someone else. I’m just theorizing here but… this screencap is very important and should not be overlooked, whatever the reason for it.