343 Industries has revealed that they aren't planning to make any more Halo spin-off titles like Halo 3: ODST. Instead, they want to concentrate on the main narrative for the franchise.
Speaking to Official Xbox Magazine, Frank O'Connon from 343 said "I can tell you we're not working on any side stories as games at the moment."
"There'll certainly be more Halo fiction entering the universe, but we've no new games to announce right now," he continued.
He also stated that while 343 Industries have no real plans to release a main Halo title in the short-term, they will be planning some interesting things for Halo's 10th anniversary. There will be numerous novels released and some more Halo: Reach map packs.