The news that Bungie had signed on with Activision came as a surprise to everyone, particularly in the wake of the Activision/Infinity Ward employee drama. Though Bungie may now be focusing on the last leg of marketing and development for Halo: Reach, most are more curious to see what their next game will be. It's hard not to imagine Activision taking advantage of their reputation by hoping they design a game within the family of 'Call of Duty Style' shooters, but knowing how hard they have worked to break away as a development studio (and how their agreement specifically states they will maintain the rights to what they make) I think we're looking at a different future for Bungie.
Though implications of the Bungie/Activision alliance are really just speculation at this point, I can't imagine that Activision would take a developer with such a large fanbase and simply have them work on one of their current ideas. In fact Bungie has already stated otherwise, explaining their alliance as one which will give them access to 'multi-platform and marketing expertise'. Having done nothing but FPS titles for a few years now, I would imagine that Bungie is really ready to try something new to flex their development muscles. Particularly since any new FPS game they make will naturally pull away from their Halo: Reach fanbase, which could make Microsoft less than thrilled.
It's not like Bungie doesn't have the experience to design different games either, having worked on the heavily-criticized Oni and notable Myth series before. While it would be completely understandable for Bungie to explore a new IP in the shooter category (it is what they're most well known for), I don't think it's unreasonable to expect a bit more exploration from the developer. Particularly if they're looking to go multiplatform, which means that they're looking to expand to a larger audience. A game with more platforming or adventure elements would certainly be a new direction, and would allow their team to step back from their traditional comfort zone and get creative.
I would imagine this is something Activision would want as well, since the Halo series is so firmly grounded to the label of Microsoft anything new would be more easily associated with their own name. Particularly if Activision has seen the new IP Bungie is looking to design, and was impressed with it's potential longevity (hence a ten year contract). There's obviously no way to know what happens behind closed doors, but certainly Activision wouldn't of signed on if they didn't think Bungie's next title wasn't significant on some level.
In the end gamers (and the rest of the industry) can only be left to speculate as to what exactly is going on between the two, and of course who will profit more from the arrangement. But simply judging from Bungie's extremely diverse development history, their constant push to develop new and useful multiplayer standards, and their desire to go multiplatform, I think we're looking at a game that's extremely different from anything they've created before. Whether or not that's a good thing will be something the we'll all find out over time.