This is not a troll post, just an opinion but~

I personally don't like the idea of DRK being a tanking class, for a number of reasons that I will mention:

1) In current content (won't mention 2.0 as anything from here will be speculation) there is only truly room for 1 tank per party for most content. With already 2 seperate tanking jobs (PLD and WAR), in comparison to other jobs (2x "mages", 2x DD's, 1x support role) - adding another job that is tank heavy won't benefit party play on a huge scale, and if anything - take too much opportunity away from the PLD and WAR mains out there.
2) MRD already has a tank job that stems from it - and that's WAR. I would rather see MRD get a heavy DD (comparable to DRG and MNK) as a secondary advanced job, as they have the base of it there already, but truthfully don't quite cut it. So personally I would rather see the DD side come out on MRD a little more, rather then a tank.
3) DRK has traditionally been a DD role (and while I am trying not to sound like an XI DRK fanboy), that's the model that I can see DRK falling into, rather then a tank role.

So in summary I believe that there is more place in this game for additional DD's, but very little room for additional tanks. If we were to look at "2nd advanced jobs" I would prefer to see a model a bit more like the following (and yes they are based off XI jobs at this stage):

MRD > DRK (DD role) or BST (Pet tank or DD pet role)
LNC > DRK (DD role) or SAM (DD or evasion/parry tank role)
PGL > NIN or THF (evasion tank/support role)
ARC > RNG (heavy DD) or Musketeer/Corsair (support)
CNJ > SMN (yes I know summoner has always been announced as an advanced job for a seperate class)
THM > RDM or "Geomancer" as the OP states (debuff/buff/support role)
GLD > BLU (solo melee/mage/tank style class)

As for geomancer, I like the concept but it sounds a bit like a RDM to me. Since we are playing an FF title (not trying to be picky) I would rather it being called a red mage (Yes I am a traditionalist). I would also rather see it stem from THM rather then CNJ. I am all for the concept, however if they were to add a 2nd "advanced job" to the classes, I would prefer to see it in a significantly different role to it's current advanced job. By that I mean BLM being a heavy DD, I would like to see THM stem into some form of support/healer style role. This way you can then bring out the magic DD out of CNJ and utilize that side of the class.

This would not only allow for more flexibility in how you like to play your base class, but the one point I will make is it will potentially allow the players with a very limited base class selection for end game, to have a higher chance at getting a slot. (Ie you can only have 1 tank in most end game content, so that MRD that isn't quite as skilled/geared to tank can step in the heavy DD shoes).