Admittedly it was difficult to find a MLP character to match up with the outrageous Franky and while Big Mac is on the far end of the personality spectrum in comparison to the Cola-powered Cyborg, I see that both have a “Big Brother” aspect about them. Both are pretty darn strong but not really the forefront fighters or aggressive attackers (Zoro-Rainbow Dash/Sanji-Applejack) but their true strength is in their constructive side.
 
Given Big Mac is the Farm boy/workhorse, he maybe knowledge on landscaping and setting up proper foundations, likewise with Franky, while he is a fighter and a weapons-creating cyborg, he is first and foremost a Shipwright and carpenter. Together, they are working together rebuilding the Apple Family Barn (yet again).
 
Interesting thing would be Big Mac and Franky having a rivalry (Traditional vs Technology) but have a mutual respect in their talents.
 

 
Note: this image is supposed to link up with the SanjixApplejack image, if it just match up properly I Guess it could stand on it own.
 

 
Edit note: made a substantial change to Big Macintosh’s positioning, was not overall pleased with the original angle but also shows how critiques are helpful in improving the image during the process.