Where do we start?
Halo Wars was made by Creative Assembly, who's developers had a background in RTS games. Bungie at that point only had major experience with FPS games so I think it's safe to say Halo Wars would have likely turned out pretty badly if Bungie had made it.
Halo 4 is the result of a continuation to try to adapt and modernise the Halo franchise to suit the current market conditions as started by Bungie with Halo: Reach. As the topic has been done to death, look at it at the developers/publishers point of view and see if you can think of an answer that would satisfy everyone whilst growing the franchise:
How can you modernise and make Halo grow without changing the formula too much that it risks alienating the original fanbase? Since Halo 3, trends in the market include:
- Regenerating health
- Progression (XP or some sort of reward system)
Of the above, Halo only had Regenerating Health. Bungie recognised this and the end result was Reach, which some agree started the cracks in the Halo community to begin with, with Halo 4 cementing it. Given that Bungie's next core game Destiny utilised a number of modern day elements (sprint, ads, regenerating health, 'progression'), I think it's safe to say Bungie would have done something similar with Halo 4 but on a modified Reach engine. Either way, Reach included Sprint and a progression system like modern games did but it sacrificed a competitive ranking system and some would argue that sprint and armour abilities were not balanced correctly for Halo's sandbox.
Ultimately, it comes down to the fact that Halo will eventually stagnate if nothing changes as there will be no new incentive for new players to join the franchise, and less incentive to some of the regular players to purchase the next instalment.