This would simplify the default configuration, but at the expense of hurting scalability with the default configuration. To help address this, we would need to make it easier to change the configuration to use PairTrees or AppleTrees.
4. Keep Pair Trees under the hood, but use them only for "path segments that don't exist"
In this scenario, Modeshape will create hidden "pairtree" nodes for the creation of any resource whose URI contains path segments that do not correspond to existing resources in the repository. These nodes exist solely to satisfy the needs of Modeshape impl internals, and are not accessible via HTTP. A GET to one of these internal nodes would return a 404
Let's suppose that the repository is entirely empty, and contains only the root resource http://example.org/fcrepo/rest/
The end result is the creation of http://example.org/fcrepo/rest/a/b/c/d/e/myResource as usual
A GET to an intermediate node http://example.org/fcrepo/rest/a/b returns a 404
Internally, modeshape has a hierarchy as usual: root->a->b->c->d→e→myResource, with nodes a, b, c, d, and e being "pair tree" nodes.
When migrating to Fedora 5, existing URI paths would not change. The only observable difference would be that pairtree nodes (path segments that were created as pairtrees) would return 404.
I think this option is complementary with (1) or (3), with the choice of (1) or (3) being reduced to "which ID generator to choose by default". This option (4) is merely saying "keep pair trees in modeshape where necessary structurally, but don't give them representations on the web any more"