Might do a supplement to Codex: Space Marines, thus making them an Ally for us. Chapter Trait would be ideally something like;
- Unorthodox Deployment: All models with the Deathwatch Chapter Trait have the Deepstrike, Infiltrate and Scout special rules. Any unit containing at least one model in Terminator armour may not Infiltrate
- Xenoshunters: Preferred Enemy: Xenos (Orks, Tyranids, Tau, Eldar, Dark Eldar, Necrons)
- Chamber Militant: Models in a detachment from Codex: Deathwatch are considered Battle-Brothers when allied with a detachment from Codex: Grey Knights or Codex: Sisters of Battle.
I'm thinking Brother-Captain Artemis for an SC, maybe Kryptmann for an Inquisitor SC, and then recycle the Captain and Ordo Xenos unit entries. As someone else mentioned, some Deathwatch relic gear (a lot of room for interesting stuff there, as Deathwatch Marines are authorised at times to use xeno-tech). They'd then just have two Troops entries, which would be the PA Kill-Team and the Terminator Kill-Team. Given Kill-Teams rarely operate with support beyond their transport, I'd say that's all they'd have. But to compensate, they'd make the Kill-Team really flexible and mutable. Ie, like the old Chapter Approved. So, like;
- Artemis (power sword+combi-weapon, some special rules relating to Tyranids)
- Kryptmann (grants bonuses against Nids, maybe a unique Warlord Trait or special rule etc)
- Generic Watch-Captain (re-use Captain profile from Marine codex, but with crazier gear options in the armoury. Also no Command squad obviously)
- Generic Watch-Epistolary (Mastery 2 default, to compensate for the Mastery 1 Codicier in the squad)
- Kill-Team: Standard Veteran profile, bolt pistol+bolter+ccw. Special Issue ammunition standard, two special/two heavy per 5 Marines in the unit. Models not armed with a special or heavy weapon can replace ccw with a melee weapon from the armoury. Can replace bolt pistols with hand flamers, plasma pistol or infernus pistol. Counter-Attack and Adamantium Will to represent the superior standard of training they go through to resist alien psykers and fight against impossible odds. I'd add Stubborn in too, given they are only deployed when required and failure isn't an option. No Sergeant, as the Watch-Captain is meant to lead the unit, hence his placement in HQ to act as the Warlord if need be.
Power-armoured Kill-Teams can also upgrade to have one Codicier (with only 1 wound), Tech-Marine and Apothecary. Wouldn't be too powerful as Riptides still nuke them to hell, but it would be cool to represent mission specialists. I'd also add the option of jump packs or Bikes for power-armoured Kill-Teams. If you choose to take jump packs or Bikes, everyone has to upgrade.
If on foot, can take Rhino, Razorback, Land Raider or Raven as dedicated transport. If with jump packs, can only take Raven. Bikers can't take any transport obviously, but Deepstrike represents them getting dropped off by a Thunderhawk.
- Terminator Kill-Team: Think Deathwing but without the Vengeful Strike (ie free swaps to alternate Terminator loadouts), and special ammo with a storm bolter profile (to give people a reason not to instantly swap out for Stormhammers). 1 heavy weapon per 5, as is normal, I'd add a conversion beamer and plasma cannon as options here (just for a bit more archeotech feel). Seperate this from the PA squad so you can clearly define mission specialists and not get people trying to mix Terminators with Bikers/jump packs and other such nonsense. Landraider or Raven as dedicated transport, they're too bulky for anything else.
No need for Elites (Deathwatch are the hand-picked elite of their respective Chapters), no Fast Attack (they operate without support usually, or tangentially to other allied forces), no Heavy either (they're a fast special forces team, no time for tanks or artillery). You can get Flyers or tanks via dedicated transports. Troop Terminators is a risk, but I think Riptides and lower cover saves mean it won't be that powerful in practise.