I'm not convinced that multiplayer is a good or useful tag. It's primary use at the moment is as a meta-tag to allow for filtering of questions concerned specifically with a multiplayer-mode in a game where that differs wildly from the games single player mode, such as the 106 questions currently tagged with both multiplayer and mass-effect-3. For most similar categories of questions, where there's a large enough number to justify it, we've been moving towards tag suffixes. Thus, I'd propose that mass-effect-3-multiplayer, which squeezes in just barely beneath the tag limit, be applied to those 106 questions (as well as the, I'm sure, several others that should have it but haven't been double tagged).
As for other possible suffix-tags, We should so probably do this with the minecraft overlap as well. I'd argue that if anything, starcraft-2 questions should simply be assumed as multiplayer and that we should be more diligent about identifying and properly tagging starcraft-2-campaign questions, if we feel the need to subdivide those at all. The rest of the overlapping tags are sufficiently small in size that I don't see any practical reason to further clutter them with hyphens.
Any objections? If not, I'll add this to the tag cleanup list, and commence burnination, and eventually, request a rename to mass-effect-3-multiplayer for the final 106 later this weekend.