So I just tried making a variation to test it out, and getting the sugar right is the hardest part. You can totally substitute whatever flavour you like, but you'll have to add or subtract sugar.
To try it out, I just made an espresso version using 2 shots of Nespresso Dharkan, and it required ALOT more sugar and condensed milk. It was too bitter without it. I also added brown sugar to the graham crust.
I didn't have any coffee Jello, but that would've gone well with it!