Because there seems to be no consensus as to people tagging these, and there's a lot of overlap, why not just lump them together?

Is there anything that one game has that the other does not that might warrant the use of both tags?

Rainbow_Dash said:
Is there anything that one game has that the other does not that might warrant the use of both tags?

You mean besides a completely free roam world, updated graphics, entirely new crafting system, whole new playable species {4 actually} that were not avilable in GW1 {where you could only play humans}

All playable species except those plantlesbians are represented in Guild Wars 1. Since it's the same universe I'd go with guild_wars_2->guild_wars.
Updated graphics and gameplay didn't change the universe.
It would be like separating fanart from different lord of the rings or Harry Potter movies.

Fancy stuff aside, it's like halo and halo 2 when just halo_(franchise) will suffice; do we need both as tags or can we just have guild_wars_(franchise) or something?

honestly, I don't see a reason to use guild_wars_(franchise) when guild_wars can mean exactly the same thing. I always thought _(franchise) was more for, say, diablo_(franchise) where there's overlap with a character name.

Either way, up to you lot.