For such a high-profile series, Halo has been fairly consistent in sticking in the first-person shooter genre. The one exception was Halo Wars, and Ensemble founder Tony Goodman says the project's departure from the norm created some strife with Bungie.
Goodman told Games Industry International that Microsoft was risk-averse to putting a strategy game on a console, and attached the Halo name to make it more marketable. "They just said, 'Why don't you just paint over what you have with Halo stuff?' But things aren't quite that simple," he said.
But that was just the tip of the iceberg. "Bungie was kind of sore about the idea," he said. "What they called it was 'the whoring out of our franchise' or something. Yeah, that didn't create a great relationship between us and Bungie; they viewed us as someone infringing [on their franchise]."
Bungie has made more positive comments about Halo Wars in the past, like AI specialist Damian Isla calling it a "very faithful adaptation" in 2008. It could be that the grumblings Ensemble heard were from different staff members, or that Bungie simply didn't want to speak publicly about its concerns. Either way, the series is back in solidly FPS territory for the foreseeable future.