Similar to #8 the only reason why Iterators (or IterableOnce in general) do not have groupBy & groupMap is because, you can not have a Map[K, Iterator[V]] without consuming the Iterator as such the values of the Map would be empty.

But, since groupBy & groupMap already consume the Iterator I would say nobody would expect the values to be a lazy collection, I would even say that most people would be ok with either List or Seq.
However, I would guess that the best would be just giving users the possibility to choose which collection to use to collect the results; this even opens the possibility to a somewhat common requirement of grouping unique values by using a Set.

So the proposal is to add the following two methods to IterableOnceOps so they are not only available on Iterator but for all collections.

def groupByTo[C, K](factory: Factory[A, C])(key: A => K): Map[K, C]
def groupMapTo[C, K, B](factory: Factory[B, C])(key: A => K)(f: A => B): Map[K, C]