Personally i'd implement the core mechanic(s) first (most games only have one or two of those), test and tweak it until it is fun in isolation and then build your game around those mechanics. (If the core isn't fun the game will most likely suck (unless the game is primarily driven by something other than gameplay, such as story)).
The core gameplay should always come before anything else. You must ask yourself what is absolutely necessary so that I can play this game. If after you have done this and the core gameplay is not fun then you know you have much more work to do before you can even start on gold plating anything else
Feature creep can be a dangerous thing early on!