Thanks for the feedback! I was hoping to have the save problem fixed with that update. I'll have to figure out what is causing the problem.
About modifying the game, I can give a few hints. There is much space left for items, entities and crafting recipes. Tiles and levels are more difficult, because the memory is already maxed out as is (that is the reason why the level size is 112x112 instead of 128x128). To add more tiles, you would need to either find more memory (e.g. use the save memory as RAM) or find a fast-to-process compression to store the tile data with.