Extension pattern in a flask controller using importlib
the idea is nice, but I have few concerns. 
Firs of all, trying to call importlib every time you are accessing the API endpoint is a bit expensive. It will be a way better if you will introduce some cache first.

You can use 2 approaches to build that cache:


Lazy, where you lookup cache first and if you cannot find the proper handler, you are trying to load it using your trick with importlib
The standard approach where you can use e.g. pkgutil.iter_modules (https://docs.python.org/2/library/pkgutil.html#pkgutil.iter_modules) and build the handler cache before you are actually start handling the API endpoints. 


I have also few notes regarding your code:


Use format functions instead of concatenating strings: 

".{}".format(endpoint)

Don't catch general exceptions (just use more specific classes than Exception class) and try not to put more then 1 statement under the try...except block.
Use pythonic naming handlerClass -> handler_class
In your handlers you can make names as read-only properties:

@property
def name(self):
    return "sample"

Try to follow PEP8 in module imports:

# System modules
import importlib 

# 3rd party
from flask import Flask, abort 

# your modules
import handlers