Any recommendations to improve this logging framework? Or is it overkill?
You don't need to re-implement MessageFormat. It is much more powerful than what your implementation provides.



You don't show us the class of LOGGER, but based on the fact that you have a Log class, I suspect you are not using a logging library. I highly recommend you look into using one instead of implementing your on custom implementation. 

If you are using a logging library, you are abstracting to much away from the library. All of the frameworks I've used support message formating and detailed exception logging (with stack traces).



The problem you are really trying to solve here is to allow yourself to reuse a number of common log message formats. This can be done with a simple set of Strings set to public static final.