Dissidia has already gap-bridged all installments and previous releases have already given us the plot element that ties them together: The Void. You could possibly say that Gilgamesh, in his unending quest for weapons to add to his collection, visits many worlds quite frequently, gets drunk and tells stories from the places he's been that barely anybody pays attention to (because he's a drunken lunatic), which subsequently results in many similarities between the worlds (i.e. characters named Cid, Wedge, Biggs, etc.)
Or (somewhat) more believable is the idea that the Void is the starting point and all realities are splintered bits from that origin, hence similar creatures, species, and stories.
Sadly, if VII and X are connected then X should never have been X but rather VII-something, unless they want to run the convuluted explanation of Gilgamesh kidnapping Shinra and dragging him to the world of VII through the Void, but then the gap-bridging title would have to be something akin to "VII=X Transworlds" or whatever and, oh boy, am I tired of thinking about this.