So, it looks like certain Forgeworld models/units are being added into the Deathwatch “cannon” of allowable choices (at least in a CAD or Allied detachment), which should provide a little more variety and flexibility to players fielding a Deathwatch list.
One of the new FW units available to the Deathwatch is the Leviathan Dreadnought as a Heavy Support option, which is nice since the only other HS choices they currently have are the different Land Raider variants, meaning that there is not too much competition for space in a CAD or Allied Detachment.
Look at the Leviathan, I think it actually fills in several of the “gaps” or weaknesses of the baseline Deathwatch line-up, particularly in terms of durability, close combat potential, and way to efficiently counter enemy Lords of War (i.e. Super-Heavies, Gargantuan Creatures, etc.) —
 DURABILITY – With AV13/13/12 (so immune to S6 and below from the front, including Krak grenades) and 4 HPs base, plus built-in Extra Armor and 4++ save and the ability to upgrade with Armoured Ceramite, the Leviathan is easily the toughest non-Super Heavy walker in 40K, with these stats basically making it a walking Land Raider in every category except having AV14 armor (and if your DW Librarian rolls on Technomancy and gets Warpmetal Armor, then you can actually push the Leviathan up to AV14/14/13 stats, making it even more difficult to damage). Importantly, the Leviathan can take a Dreadnought Drop Pod, not only saving it from having to “foot slog” across the table to get into the action (and take multiple turns of enemy shooting along the way), but also providing an “outer skin” of protection via the Drop Pod’s Armor and Shrouding that the enemy has to remove first before they can actually begin damaging the Dreadnought itself. Just for reference, with Armoured Ceramite (basically a mandatory upgrade, IMHO), on average it takes the following to destroy a Leviathan:
-24 x BS4 Lascannon or Bright Lance/Dark Lance shots
-36 x BS4 Meltagun/Multimelta or Krak Missile shots
-18 x BS4 Railgun shots
-64 x BS3 High-Yield Missile Pod shots
-72 x BS4 Gauss weapon shots
-36 x BS4 Grav Cannon/Amp shots
-14 x BS4 Haywire Blaster shots
As you can see, it take some VERY significant firepower to bring down a Leviathan Dreadnought, especially when it is landing in the face of your army via Drop Pod turn 1. This is particularly relevant for a Deathwatch force because, number one, enemy armies cannot afford to simply ignore it because of its very potent shooting and close combat potential (more on this below) and, number two, because it can absorb a ton of firepower and attention away from the rest of your expensive and somewhat fragile DW squads/Kill Teams. Put another way, the Leviathan Dreadnought makes an excellent “anvil” unit to complement the “glass hammer” nature of most of the rest of the Deathwatch faction.
 COMBAT ABILITIES – While most dreadnoughts these days are actually quite dangerous in the assault phase, with 4 x S10 AP2 attacks at initiative in their base profile, the Leviathan takes this threat potential to an entirely different level. With the durability described above (i.e. AV 13, 4 HPs, 4++, and immunity to Melta rule), it can easily get close in with the enemy and shrug off everything from melta shots to Power Fist attacks, meaning your opponent will have to devote significant resources to dealing with it. On top of this, it has WS5, 4A base, and gains I5 and two Hammer of Wrath attacks (both at S8) on the charge, plus it has Move Through Cover and built-in Frag Grenades, mitigating two potential pitfalls when attempting to charge. Finally, for 1MB cost it can switch out one of its Siege Claws for a Siege Drill, giving you the flexibility to use the Siege Drill (S10 AP2, Wrecker, Armourbane) to rip down everything from fortifications to Imperial Knights OR switch to the Siege Claw (S10 AP2, Wrecker, Severing Cut – Every time an Unsaved Wounds is inflicted, on a 4+, inflict ANOTHER D3 wounds using the same profile) to devastate enemy characters, MCs, or even GMCs like Wraithknights and Stormsurges.
Combine all this and you have basically a “mini-Lord of War” style walker that can arrive turn 1 via Drop Pod and instantly begin threatening the enemy battle-line (NOTE: The Leviathan also comes stock with BS5, two Meltaguns, two Heavy Flamers, and the option to take up to 3 H/K Missiles at 3 MBs a piece, making a quite respectable at shooting as well). As I mentioned before, this synergizes very well with the rest of the Deathwatch, as they are able to apply very potent firepower themselves, particularly when the enemy’s attention is focused on the Leviathan. Described another way, because of how deadly it is, the Leviathan is the ultimate “distraction Carnifex” and can make quite a difference, both psychologically and in terms of raw combat power, in any Deathwatch force.
 LORD OF WAR COUNTER – I have already kind on hit on this, but one of the areas that the Deathwatch faction, surprisingly, have some difficulty with is dealing with enemy LOWs, short of suicidal Melta/Grav drops which can be risky and which the low model count Chamber Militant of the Ordos Xenos cannot really afford (unlike, say, Astra Militarum, which can throw away suicidal melta command squads like it is cool and not miss a beat). Even their Mission Tactics and Kill Team special rules, which cover pretty much every type of unit from the FOC, do not have a category for bonuses against Lord of War choices, meaning that Deathwatch don’t get any bonuses to off-set this weakness against the Super Heavies, Gargantuan Monstrous Creatures, Draigos, Ghazkhulls, etc. of the 40K universe. This is where the Leviathan comes in. When it first drops in, it can use its BS5 and meltaguns, plus and H/K missiles, to put a couple of wounds on an enemy LOW. Then, on subsequent turns, if it can get into combat with the LOW choice, the S10 AP2 attacks at WS5 and I5 (on the charge), plus the bonus damage via either Armourbane or Severing Cut, mean that the Leviathan can efficiently bring down even the mightiest Lord of Skulls, Wraithknight, and/or Ork Stompa, in turn removing that threat from the rest of the DW army and also freeing those forces to use their combat power on other, more viable, targets. Once again, I can see this as “force-multiplier” for DW commanders that goes beyond just the direct combat power of the Leviathan and which enables the entire force to function more effectively/efficiently as a whole.
While I have not addressed the additional shooting options for the Leviathan here (i.e. the Storm Cannon and the Cyclonic Melta Lance), I think the Deathwatch already has plenty of capability in these areas from its own weapons (i.e. Frag Cannons, Meltas and combi-Meltas, CMLs on Terminators, etc.), so I have focused on the “close combat variant” of the Leviathan in terms of integrating it into the DW faction as a whole.
Since it is a Relic of the Armory, you can only take one per army unless you have a Keeper of the Relics, which FW has not “assigned” for the Deathwatch yet; hopefully they will, since fielding two or even three Leviathans as Heavy Support choices in a Deathwatch CAD would be highly effective, I think, alongside the DW Veteran squads and other units available to this faction.
Taken all together, I think the Leviathan Dreadnought is an excellent addition to the Deathwatch and, if used by a savvy commander, can mitigate many of their weaknesses while also enhancing the strengths of other units in the army.