CTools has already implemented the pluggability that we want. CTools provides the
content_type plugin which have sub implementations like:
- Panel panes
- Page elements (site name, slogan etc)
- Custom text
I suggest that the ESI module relies completely on the existing CTools plugin
content_type. Adding the dependency on CTools is just a win, since it will exist on all D7 sites anyway, now when Views 3 depends on it.
The ESI code base would be rather small and easy to maintain. I'll see if I can make a stab at this one soon.