I know this will probably sound odd, but I would advise that you don't spend too much time on your editor and forget to write your game.. or build an editor which becomes a feature creep monster and takes time away from developing your game. Time and Creative energy are a premium, don't waste them.

I spent a long time working on an editor for a game I was writing called Anomalous, in the end, I hadn't considered some things and it became too much work to fix.. the editor was cool though, it used the game engine itself to render tiles and sprites, saved files in XML and was quite powerful, I'd got a list of additional features I was going to add but I got overwhelmed by the size of the monster I'd created. Also it was very specific and could not be used on any other games so was in effect, a complete waste of time. I should have used something else.

I would consider using an existing editor these days, if I could get away with using TileStudio, or Mappy, then I would.. even if it meant writing a conversion utility to convert between their format and my own... I'm sure even things like Milkshape or Xara could be used as a level editor if they were used in the right way and files were exported into a format which could be processed.

Re-inventing the wheel can be fun and sometimes seems like it's the only way, but at the end of the day, you've just made a wheel, very similar to lots of other wheels.. the rest of the car is still to be built.