I would like to see a sequel series written, but set in the future. So, like, not right after the ramming of the blade ship. But instead, introduce brand new characters either on Earth, in space, or both. Have it be 2017, with the ending of the last book falling under the same chronoligical timeline. That would put the gang somewhere in their 30's.
Maybe there is another threat out there, and a young group of teens needs to step up and stop them some other way. Maybe animal morphing isn't as common and instead there are a number of alien species that can be acquired for use in battle. Maybe Elimist is still playing his games and interfering while not interfering.
And maybe make vague references to what happened in the past, with the group ramming the blade ship. Like, don't spell it out exactly but point to historical events that were impacted by the final pages of book 54. Is The One still out there? Is that the threat, finally making it's way to Earth space?
Maybe they live and make an appearance somewhere deep into the new series. Maybe it is confirmed that the action of ramming the blade ship caused a chain reaction of positive events to unfold, although they all died in the process. Maybe Cassie goes searching for them, and ends up like Elfangor, crash-landing back on Earth somehow, giving her dying breath to a group of teens and telling them to FIGHT.
I just think someone could write a very successful book series today, in the Animorphs universe, under some of the circumstances outlined above. Without having to simply take over directly after book 54 ends, and without even including the entire main cast of characters.