I would just like to reiterate the value of creating a plug-in based editor. Jeff Ward pointed me in this direction a few years ago when I started working on an editor.
Creating such an editor will allow you to remain agile in development. Inserting a new component into the system should be a painless experience. Requirements will come up where you didn't expect them and having this kind of architecture will save you time in the long run.
And, tbh, from a developer stand-point -- this was pretty fun to experiment with. :)
Easy-to-use Event Editor
I wouldn't want to see one window with a thousand checkboxes, comboboxes, etc. Give me one window that does one thing. Imagine right clicking on a door and selecting add event. It recognizes it's a door and opens a 'Door' dialog. Well, what do we do with doors? We go places. Have a drop down of other positions the player can go. KISS
It would be handy if I could start designing a dungeon, or even island, from a random start-point.
Improved Tile Placement
Say I started drawing the exterior of a castle or the coastline of an island. Imagine, for the island, that I needed to go around the island with the coast tile. If I could hold shift and set the coast tile all around the island, that would be slick.
What I use for one game, I'd like to use for another game. Create a way for me to store my characters, items, monsters, spells, etc into a database.