For some weeks in 2012, "arbitrary" gTLDs could be applied for at ICANN:
On June 20, 2011 ICANN's board voted to end most restrictions on the generic top-level domain names (gTLD) from the 22 currently available. Companies and organizations will be able to choose essentially arbitrary top-level Internet domains.
According to a search on https://gtldresult.icann.org/, no one tried to register
.onion in the last application period. I’m not sure, but I guess ICANN will open the application system again, sometime.
Does the Tor project plan to register the
.onion TLD with ICANN?
Would the Tor project have any privilege to claim this TLD, so that no one else may register it beforehand?
If someone else would register
.onion and create domains matching those of hidden services, we’d have two incompatible systems.
(EDIT: see also the related questions "How would a registration of the .onion gtld impact the tor network?" and "What happens when I visit a .onion website".)