Base Distribution

FYI: Technically this is quite easy to achieve with a new action in Frontend\NodeController:

1public function createAction(PersistentNodeInterface $parentNode, NodeTemplate $newNode) {
2    $createdNode = $parentNode->createNodeFromTemplate($newNode);
3    $this->redirect('show', NULL, NULL, array('node' => $createdNode));
4}

In conjunction with #27117 this should already work.

The problem: Security!
Obviously not Everybody should be able to create nodes. Also we can't restrict this to authenticated users (this would not work for the described scenario where anonymous users are allowed to post comments).

For this to be implemented we probably need some kind of "node restrictions" (e.g. nodes of type X can only be inserted below some/path by role Y).
At some point we need that anyways for Neos..

at least

nodes of type X

could be implemented with something like a (todo) instanceof runtime constraint operator which would be useful to have anyways.

Or, the runtime constraint handling will have a new feature where arguments must satisfy particular, more fine grained checks, just like

1resources:
2  methods:
3    Foo_Bar_FooBar: 'method(Foo\Bar->SomeAction(product.owner = current.securityContext.party && newNode instanceof Acme\Foobar\Node\CommentTemplate && parentNode satisfies Acme\Foobar\Constraint\CommentNodeCreationConstraint))
4

the product.owner part is what we already know; the instanceof would be the mentioned operator which checks for the argument being instanceof the given class/interface (just like within); and the last one would be something like the mentioned user-definable constraint satisfaction.