Geez, what's with all the multiplayer votes? It's going to be very difficult to implement, and who knows how good any sort of implementation is going to be. I personally think that should wait.
Modularity and debugging are certainly high on my list, though I do recommend tile map support, which I voted for. Support for the most widely-used tilemapping editors would go a long, long way, and as well, Tiled recently implemented a Terrain tool, which enables easy transitions between types of tiles (water, sand, grass, dirt, stone, etc). As someone who works with tiles, tilemap support would be an absolute godsend.
I'd definitely say tilemaps, debugging and modularity should be done first. Multiplayer can come much, much later.