If I would run a 12-box, it would probably look like this:
GRP1
Shadowknight
Illu
Troub
Warlock
Inquis
Wizard/Mystic
GRP2
Monk (Conjy as secondary)
Coercer
Dirge
Brigand
Inquis
Defiler
My thoughts behind this setup:
Group 1:
This is the group (with mystic) that I currently run. It's a setup with a lot of DPS potential and with a wizard (which I'm currently working on) I'll be able to push some really decent numbers. In this hypothetical 2 group scenario, it would be the offtank / DPS group. With the gear I have on it, one healer should be enough for most stuff, even more so with the dps these two groups would dish out.
The group will offer a crapload of temp buffs for DPS to the price of some survivability, nothing that can't be handled though.
Group 2:
Basicallly an MT group. The monk, dirge and defiler combo will offer a very solid combo for survivability. Inquis over templar will offer more DPS to the cost of a small variety of debuffs and some survivability. The DPS increase would most likely be essential here though as the monk would not have a proper hate transfer. 12% from the warlock, can't recall exactly how much from the coercer and 5 more per cent from the monk avoid would on the other hand stack up to a fairly decent amount with the additional possibility of transferring hate from G1 via the new troub AA.
Coercer - extra healing potential for the group, massive power potential, variety in debuff stacking, hate ... Don't think I'll have to explain further, do I?
Brigand - sure, less DPS than an assassin - which was the first that crossed my mind - and of course no hate transfer, but instead I'll gain debuffs of all sorts! I tend to think that many people boxing - as I said, I think, can't know for sure - underestimate the difference debuffs do for the DPS and survivability. I believe that the complexity of the assassins' "stab chains" (not very complex when playing manually, but botted they tend to fail way too often, but again, that's my experience) would render the gain in DPS from that slot alone less than the brigand's lower individual DPS but boost to every other slot.
The conjy on the monk account would go into G1 - switched with the SK - for AE-stuff that only requires one tank.
Beastlords ... Mine is still level one, so I don't know much about what they have to offer - I know some of course as people discuss it alot these days - but I can't help thinking that they are nothing but a jack of all (or well, some) trades but master of none. In the setup above, I've tried to build it around the masters of each trade:
SK - Somewhat squishy these days, but well geared it's very much capable of tanking most stuff. Pally would offer slight more AE aggro, but let's simply state the fact that I have no issue with the aggro today on my SK with the parses I do on my warlock (see the parse thread for reference). The monk would of course have a little more problem on AE-stuff, but there's always to possibility to toss the SK into the tanking role for those fights.
Warlock - master of AE DPS, very high single target DPS. Rest my case.
Wizard - Debatable if it's a master of it's trade, but it's very good ST DPS, high AE DPS and would make optimal use of the buffs in the group.
Troub - UT for the warlock, rest my case.

Of course other stuff as well, but yeah ...
Illu - TC, IA, AoI, Synergism, temp buffs, power regen ... heck, you know the drill. A good boost for the DPS and provides a decent amount of DPS itself.
Inquis - only thing it lacks in a DPS group is wards. Buffs, debuffs, a good portion of DPS for a healer, double group cures, stifle immunity ... rest my case.
Mystic - with a defiler in the other group, the mystic provides the wards the inquis doesn't have, decent dps itself and more different debuffs.
(Conjy) - Elemental Toxicity omg! Raidwide buffs, some minor mitigation debuffs, DPS. The drawback is of course the pet's survivability, but I'm sure that could be solved with some innovativity with stone skins etc.
Monk - Survivability and DPS, nice boost for the melee classes in the group ... I guess a bruiser would do the job as well.
Dirge - stoneskin, more debuffs, hate, more buffs, rezzes ... Don't think I'll have to say more really.
Coercer - as already mentioned: Power, coercive healing, debuffs, buffs ...
Defiler - more debuffs and of course massive wards. Not insanely much more than the mystic, but diversity in debuffs is never wrong!
Inquis - only chosen over a templar here for the boost in DPS really. Double group cure is of course nice to have, but not essential in a 2 healer group. With a lesser geared tank, go with the templar, heck ... there's always the possibility to have one of each on this account to rotate depending on content.
Brigand - master of debuffs! Defensive debuffs, offensive debuffs ... Debuffs almost as much HP as an Assassin does, but its other debuffs will gimp the mob even more (less heals required - more dps from the healers and of course less risk of sudden damage spikes causing a whipe) and more DPS to all other classes in the raid (less time for the shit to hit the (in-)famous fan).
With this 12-man setup, replacing either class with a beastlord will make you lose out on at least one master of its trade. If you (Jagger) still feel the absolute need of tossing it in somewhere, I'd say you should toss it in instead of the conjuror which here is the weakest link in a full DPS setup.
As you have a templar geared and fit for fight, go for it as the MT healer (you can always roll an inq later on if you think it's worth it). The guardian ... well, I have to admit I have a guardian as well, but it's just because it's an old toon and it's on a different account than my other tanks (for 2-tank encounters). Sure, if I was you, I'd use it until I had geared up a brawler on that account, but that's about it. With a 12-man setup, you'll have such potential of killing harder stuff that having a guard there would be one of the first bottle necks. But when it all comes around, it's a matter of what you gain versus the effort put into it. It's yet another toon to grind and gear up, and I can't tell you if you'll think it's worth it for that extra ounce of survivability and DPS. After all, guardian isn't a bad class, it's just that there are better options out there.
Well, /rant off, I hope it provided more thoughts rather than a "tl;dnr". :lol: