I used a double-linked list of gap buffers (each in its own fixed length block that can swapped out to disk) for JOE. It works great on large files, but still I would start with rope these days.
Gap buffer was appealing on really slow machines, where you are counting cycles on each key-press. If the gap is at the right position, the cycle count is very low. But you can probably do the same even with a tree-structure: you need to keep a pointer to the leaf in the abstract pointer used for the cursor.
Also I would tie in the undo system to the data structure if possible. Rope does this with copy-on-write. Every version of the file could be a different top-node, and most middle and leaf nodes would be shared between revisions.
Gap buffer was appealing on really slow machines, where you are counting cycles on each key-press. If the gap is at the right position, the cycle count is very low. But you can probably do the same even with a tree-structure: you need to keep a pointer to the leaf in the abstract pointer used for the cursor.
Also I would tie in the undo system to the data structure if possible. Rope does this with copy-on-write. Every version of the file could be a different top-node, and most middle and leaf nodes would be shared between revisions.