This is something that I have been working on for the past two weeks or so. The intention is for this to be like the SMW Customizer but for YI, allowing the user to make a variety of tweaks. Python 3 is required for this program.
The aim is for this (in version 1.0) to contain everything in the Tweak Map, which I have been moderating. It's currently in Version 0.1 so only contains a relatively small number of tweaks but it will be updated throughout C3. Let me know if you have any things that you would like to be added to this or any bugs/feedback.
A very nice initiative. Yoshi Island is in need for new tools and that one opens a very good potential for customizing the enemies. Consider also implementing part of the Tweaker features which is more specialized into sprites.
I tried opening your tool myself. Apparently it does not like the *.sfc file extension, it threats like "not 1.0 YI ROM". Consider adding a different treatment for each different "wrongROM" situation (the file extension is not valid, the ROM has the wrong size or title, the YI appears to be 1.1 instead of 1.0, etc.); the more you detail an error on a tool, the better is to the user know what happened.
However, for some reason after renaming for .smc, the GUI appears for a split second and the program is closed. Nothing is output on the console. My Python version is 3.6.0 running on Windows 10.
I have gotten it to work in IDLE. The first thing you do is to open it in IDLE (either within IDLE or through "Open With...") and then go to Run > Run Module or press F5. That should open YI Customiser properly (although there is a warning that an unexpected error occurs if you close the tool without selecting a ROM but that is nothing major).
As someone who's been using the SMW Customizer for a while and experiencing how useful it is, I'm glad that a similar tool can be used for the YI-hacking scene. Even if's still in early development, I'm sure this'll have great use in this community.
Thanks- BTD6_maker- for this tool, and good work on making this.