After my last post, I realized that it sounds like I'm saying that it would be a good idea for an IP such as Final Fantasy or Gears of War to come out with a bizarre new style of game instead of a normal sequel.
I'm definitely not suggesting that! Those IPs are not built to support that kind of variation. If you came out with Gears of War the RPG, it would probably be considered a very awkward move.
If IPs like these do come out with games from outside their previous style, they tend to be fairly conservative and standard. Chances are very high that the Halo RTS will not have any amazing new dynamics in it: it'll basically feel similar to most other RTS on the market. I don't mean boring - I just mean that the mechanics will be very similar to those things that RTS fans expect.
When I wonder if it's possible to have an IP that lets you experiment with very new and unusual games as sequels, I'm not talking about any existing IPs. I'm talking about whether you can come up with an IP that would allow that.
It would need to have a specific set of attributes. For example, it would need to be diverse enough to allow for a variety of gameplay styles. Halo is better at this than Final Fantasy, because Halo concentrated on developing its non-gameplay aspects across multiple games, while Final Fantasy always develops new non-gameplay aspects. This means that Halo can abandon their typical play style and still feel like Halo so long as they keep those assets in play.
I don't think Halo is a really great example, though, because the Halo universe is very confined. It's not just a matter of how many different styles of games it can support, but also how many different styles of... narrative, for lack of a better word. You'll never get a Halo game that is about falling to your darker urges, like in Star Wars. It's just not supported by the nature of the game, and if you published such a game, it would be heralded as "a new direction for the Halo franchise". It would be hard to do an adventurous game (whether an adventure or an open-worldish RPG) because the IP is completely militarized: the lives and adventures outside of the military are not part of the Halo IP, even though they obviously must exist in a technical sense. Again, you could make a Halo the Adventure game, but it would feel very un-Halolike.
So the question is...
Can you imagine an IP and an approach that would allow you to develop radically different games from sequel to sequel without ever losing the "feel" of the IP?