Urr
Depends on your situation, using other tools which allow you write custom plugins may be a better choice for you. IMO, spine is currently the best choice for making 2d game animation. I noticed other options which is not that bad if you want extensibility more than the provided features.
If your are making game with Unity, AnyPortrait from unity asset store is worth a look. If I remember correctly, It came with full source code so you can do whatever you want to make it better. However, it seems to lack certain features like skin. I bought it a long time ago during a sale but never have time to get into detail but its so cheap that I don't feel bad putting it aside.
Another interesting alternative is Moho Pro. According to their website, you can write custom script to personalize it. To be honest with you, I never tried it but it show some interesting features on their website that I really want them to be available in Spine so it got my attention. However, its expensive if you want to keep you version up-to-date. The approach to integrate to game engine is also different. It use standard fbx file which allow them to support game engine without custom runtime library. However, some special features that is provided by the spine runtime will not be possible using fbx.
@Everyone
Regarding the topic about Spine plugin support, you guys are ... asking for too much. You don't need full feature well defined API to create tools for spine if you don't mind your tool floating outside the spine editor window. In opinion, it would be great if plugin support is available. However, as I suggested a long time ago, a lot of things can already be done IF ...
- Spine Editor can have a setting to make it save/load the project using the current json import/export format as default. (parsable text file is the key point)
- have another setting to have Spine auto detect project file change and auto reload the project.
With these 2 very simple changes, we can write external tools to parse and update the project and have spine auto reload and apply with the changes.
In fact, we can already do this kind of patch using the import/export feature to extend the spine editor but the workflow is not smooth without the 2 suggestions mentioned above. If spine team really want to help regarding the extensibility aspect for the spine editor, I think this little change could be a good start because it is really an easy task with big gain.
p.s. I am just saying what can be improved easily, it doesn't mean I am making a feature request. I am not!