Strongly believe it will be use aspects from Mummy and Gringotts but how much of that area will they use where the Dragons are and will Flight of the Hippogriff stay?
Flight of the Hippogriff will not be moved or removed. It is staying right where it is.
There's been no confirmation of absolutely anything regarding a Dragons replacement. Rumors
around the web have been for a new type of dark ride with some outside elements that may include an interactive wand/magic casting ability. It could use a Floo powder premise that allows us to teleport between famous locations in the books/movies including, but not limited to the Forbidden Forest. These rumors are partly based on
patents recently filed by Universal, (however parts of this have shown up in possible Zelda/Nintendo Land concept art so who knows if they were ever meant for Potter at all.)
New rumors,
originating on this forum, are pointing towards a different (hyper) coaster replacing dragons as an extension of a Lost Continent overlay/replacement. This is still completely unsubstantiated and appears to be based on rumors heard through a friend who knows someone (hearsay.)
The area currently used for Dragon Challenge is close in size to Diagon Alley, so it's entirely possible that an extension of Hogsmeade could be created to coincide with the opening of a new attraction, with new shops and dining options. Given the space, they could even add a second new attraction in the space, maybe another kiddie ride to balance out the park. But all of that, and I mean ALL of it is just us guessing based on size on proximity to the area.
I usually get some inside info on new attractions at Universal, and personally have no details on a Dragon replacement myself. Perhaps because all of this is just made up, being kept well under wraps at this time, or plans are still very much in flux. Until recently much of the rumors have been from Team Members saying the coaster is closing in the fall... However, that was a rumor passed around from LAST FALL as well, so take it all with a grain of salt.