People that say there isn't a big enough fanbase for Animorphs forget the fact that in the future there may be a diehard fan that wants to make the movie as much as we do. You don't need a big fanbase, all you need is one fan committed to doing it. At this point the movie would be a resurrection of the series, most of the moviegoes would be newcomers anyway.
But anyway, its good to comment on people's ideas but don't turn this into another "it's never gonna happen, so stop dreaming!" threads. This is all for fun.
I would go animated (japanese anime style would be very fun), change the timeline from the 90's to current day and for the most part remain faithful to the character and their stories, although the plot would have to be streamlined and tweaked in ways.