Dream.scala

object Protocol extends TopicOneSupport with TopicTwoSupport {
  /*can't be sealed now*/ trait PMessage
}

Protocol.scala

object Protocol{
  sealed trait PMessage
  
  case class TopicOne1(id: String) extends PMessage
  case class TopicOne2(id: String) extends PMessage
  case class TopicOne3(id: String) extends PMessage
  // and on and on....
  
  case class TopicTwo(id: String) extends PMessage
  case class TopicTwo2(id: String) extends PMessage
  case class TopicTwo3(id: String) extends PMessage
  // and on and on....
}

TopicOneSupport.scala

trait TopicOneSupport {
  this: Protocol.type => 
  case class TopicOne1(id: String) extends PMessage
  case class TopicOne2(id: String) extends PMessage
  case class TopicOne3(id: String) extends PMessage
  // and on and on....
}

TopicTwoSupport.scala

trait TopicTwoSupport {
  this: Protocol.type => 
  case class TopicTwo(id: String) extends PMessage
  case class TopicTwo2(id: String) extends PMessage
  case class TopicTwo3(id: String) extends PMessage
  // and on and on....
}