It's Easter. There are a kerfillion people in the church. Communion is probably going to take kind of a while. You know what's not your friend here?
A short motet.
Even two short motets will not do it. You have to fill a lot of time. Fortunately, literally centuries of composers have built an arsenal of ecclesiastical music, which includes a large number of pieces appropriate for the situation, some of which are quite long. Or you can plan for about six short ones, and cut them if there's suddenly a rash of Easter truancy.
Your organist is not that good at improvisation, so please save us.