It's crazy prediction time!
I wouldn't have been sure of this claim when MP4 was first announced. There was a new studio involved and Nintendo wouldn't be sure of their longevity. Indeed, the were so unsure of the developer that they pulled them off the project completely. However, as Retro Studios has taken over as principle developer, I think this idea is almost guaranteed.
First, I understand that the team that is making this game shares next to no DNA with the original Prime Trilogy dev team. I'm sure this point will come up again and again in rebuttals to this prediction, and they're fair. Just know that as the OP I'm aware of this fact, but even Nintendo themselves must feel that it isn't that big of a deal as they've basically gambled on Retro in a way they've never gambled on a franchise before (other titles were certainly gambles, but not announced years prior after a previous attempt had been scrapped).
The basis of my prediction is two fold. One: Retro doesn't seem to mind going back to the same title to try out new ideas. Metroid Prime didn't have to have a sequel. In fact, prior to its release, fandom was kind of up in arms about the direction the title was taken (as Nintendo fans usually are). Retro had the insight to leave a cliff hanger ending to the title, just in case the game warranted a follow up. And once they got that follow up they made sure to give themselves an opportunity to do one more (and even more, considering where we are now). Likewise, even though Donkey Kong Country Returns was a great title, many, many people were disappointed that they weren't working on a Metroid title and they still went back for another helping of Kong. Obviously this team doesn't get bored working with the same assets, characters, environments, playstyle, etc. They even seem to relish in it.
Second, the sheer amount of money going into this project, especially given the canned false start, suggests that Nintendo themselves are going to want to get a little extra bang for their buck. The development of a game like Metroid can't be cheap. Scenario and cut scene development alone must be quite expensive, let alone asset and engine creation. So why not go into this title with that being a given. Each asset and animation they create in this title will have a guaranteed reusability and that will allow the team and Nintendo to feel more comfortable with allowing the dev process to tick up the dollars in production without as much fear as they normally might have.
Metroid titles don't sell well. Metroid fans are ravenous and are always asking for the next game from Nintendo. This idea of a premeditated trilogy would both help to appease Metroid fans (a vocal, and somewhat whiny bunch if I can speak for myself) for the decade to come as well as ensure that while one game might not net a profit, the recycling of assets should esure that over three titles that the project becomes at least somewhat of a profit machine.
Of course, if Metroid Prime 4 sucks, which we all know is a possibility, any further ideas for sequels could be scrapped and we could be left with a stand alone title anyway. Somehow I don't see that happening, though.
And one other thing: Metroid Prime 4 will not be the title when this game is revealed for the first time. It will have a new subheading and that will be the first indication that we are in for a longer ride than just a sequel to Metroid Prime 3.
SW-
