Time that runs on more than one dimension is an unusual concept, since it's known by all interstellar cultures that it's impossible to move faster than light without exploiting loopholes that also allow for time travel, this is commonly explained as a result of all matter progressing through time not as frames in an animation but as extremely loose lines through the universe that occasionally overlap with each other. All backwards time travel can simply be described as exploiting this to make the lines move backwards. This is harder than it looks because the vacuum of space is filled with a "dimensional wind" of particles that very briefly pop into existence long enough to force matter to continue moving in the same general direction of everything in the brane. This means that all time travel requires tampering through the volume of hyperspace between all known branes.
One old theory speculated that individual progressions of time branch off into other alternate universes. All attempts at "sideways time travel" to enter these or by making a successful attempt at time travel prevent its own success, have been failures. It's difficult to tell which would be worse, the implications of this "Many Worlds" model, where nothing is truly unique, or the current model, which is that that all of time is part of a single web that can never change. The greatest nail in the coffin of the Many Worlds model was that Infokinesis cannot sense any alternate possible futures.
The greatest known threat posed by time travel would be the creation of a Nonlinear Intelligence, a being whose mental lack of bounds from the flow of time would give it a capacity for computation capable of dominating anything else instantly.