It looks like they are adding a new difficulty and discovery tour I read. As for new trophies, they didn't specify. I would not mind new trophies for the discovery mode though. I am hoping no new difficulty trophies (I suck as is at the combat haha). The update looks like it will drop 9/7 (tomorrow at the time of writing this). We will see. As far as a difficulty trophy, the AC games have never had that sort of thing, mainline games that is, so probably not, but you never know!
EDIT: River raids will be included as well and we didn't see any new trophies for the first map pack. Discovery tour isn't until later this Fall I read some more.