Halo 5 in 2015 seems reasonable to me.
If the 2014 Halo for Xbox One was an ODST-like expansion, I imagine it would be slightly cheaper, and I don't imagine it would be so big a drain as to derail Halo 5's production.
*Incoming rampant speculations*
Problem with all this guesswork is we don't have a clear idea what 343 is capable of as a studio now. With ODST, a lot of the dev team were the people slated to work on Chronicles. They were "free" resources in a sense that they could throw on the project without adjusting Reach's timeframe, or that of Destiny.
We know that back in 2007-8, 343 Industries was basically a half-dozen people. We know that there were as many as 350 people including contractors who worked on the game;[1][2] I haven't seen a reliable estimate for how large the team is now, however.
We do know that a number of key campaign- and multiplayer-related staff have left, but we haven't seen as much about the hirings, and we can't really judge timetables from that either--for example, Halo 4's writer Chris Schlerf left just recently[3] but he's credited for the upcoming Arby-related comic series so I don't think he would have started on any game work.
So all the guesses about how much time doing a project would take 343, etc., is sort of a fool's errand (but one we all love to play!) without knowing more about how the studio is structured and staffed post-release.
Like many people here, I firmly subscribe to the "a game is late..." mantra, but I don't think we can say much about the studio's capabilities to deliver a quality title going forward since they're so new and maturing. I do hope that in the coming installments they're able to consolidate more of the periphery of the game experience into themselves; while Certain Affinity is awesome and has done some excellent work, Forge clearly suffered by being spun out as a dev chore, and I think that's partially unavoidable.
[1]
http://www.joystiq.com/2013/03/29/what-went-wrong-with-halo-4s-prometheans/
[2]
http://www.gamasutra.com/view/feature/191234/making_halo_4_a_story_about_.php
[3]
http://teambeyond.net/chris-schlerf-leaves-343i/#sthash.b4CdQSUL.dpbs