Here's some of my speculation, which'll most likely be discredited later on but still.
The major implication of the teaser is that a time bomb will be involved somehow. However, this could be a metaphorical time bomb, a countdown to some big event that involves Amuro.
We've already seen him fight Akai in Movie 20, and we've seen him trying to hunt down Akai in the Second Bourbon Arc, so this new movie probably won't be about that. It'll most likely be about something else. My guess is that this movie will explore Amuro's past.
The first is his briefly hinted at connection to Elena Miyano. Second is his connection to Wataru Date, third is his connection to Scotch. Movie 22, I think, will explore one of these three elements in depth.
This may create a situation in which this movie reveals important information about the character in the canon, but that isn't so unreasonable given that Movie 21 presumably explains the connection between Heiji and Momiji before such info has been provided in the manga.
I'm really hoping for an Elena story, and this is fairly plausible given that the manga's recently been peddling in some "Akai family lived in the UK" weirdness, and Elena's from the UK. It's possible, if not probable, that in between now and April 2018 the manga will see a resolution to this incomplete and kinda perplexing Akai backstory, which might also reveal new tidbits about Elena and her husband and bring them back into the spotlight.