Add something like

    default Collection<E> Collection.asUnmodifiable()

as a default method to the Collection interface that returns an unmodifiable-wrapped instance of that Collection. Covariant overrides would be added for all the collection interfaces: List, Set, Map, Sorted{Map,Set}, Navigable{Map,Set}; and also EnumSet and EnumMap.

This makes it easier to chain. For example, consider

    List<Type> = input.stream()
        .map(foo)
        .filter(bar)
        .collect(toList());

To make this unmodifiable, one would have to do

    List<Type> = Collections.unmodifiableList(input.stream()
        .map(foo)
        .filter(bar)
        .collect(toList()));

or

    List<Type> = input.stream()
        .map(foo)
        .filter(bar)
        .collect(collectingAndThen(toList(), Collections::unmodifiableList);

It would be nicer to do

    List<Type> = input.stream()
        .map(foo)
        .filter(bar)
        .collect(toList())
        .asUnmodifiable();

and chaining additional operations to the end would be easier.