Epic Tab!!!!! *fist pump* ;-)
Please, pleasepleaseplease don't make guild achievements retroactive. Hear me out: Game activity is "spotty" as it is right now. The guilds under 100 that's AREN'T struggling are truly few and far between.
A. It's better for the current players if THEY have to be the ones to work and earn up that shiny guild medal (or however you display the achievement). Keep the CURRENT players active and involved.
B. Past players really shouldn't matter. It's not what guilds have done in the past that keeps them alive and well, it's their current activity.
C. It'll add a fun element/twist to the game to see which guilds unlock what new achievements there are first. Rather than simply log in and ta-da! look we've earned a Top 10 medal for who-knows-what several years ago... guilds might actually have to hunt titans or deposit gold or whatever and then LEARN that doing so earned them a medal.
I'll go with it, either way, but I think it's just more fun if it's like the GUILDS are starting from scratch (though we can't re-start our characters).
+1, give all of us something to work towards, nothing retroactive is the best option to spur guild camaraderie and team work. It will surely get people playing more often as well. What I did last year for my guild was already rewarded by my guildmates continuing to put up with me today, give me a reason to keep playing!