So just some ideas here on things you could run:
Inquisitor Lord w/Retinue.
Deathwatch Kill Team.
Adeptus Arbites Squads (you know, the Imperiums version of Judge Dredd).
Zealots. Call them whatever you want, religious help or angry citizens or conscripts.
You can easily have a couple of shooty units on the board with some crazy nutters running ahead of their police tenders with the above.
You can run a supplmental force of deathwatch, designed around:
Tactical squads in drop pods, and dreads in drop pods.
Toss in some cheap marine biker squads for tank-busting (116 or so for 3 bikers with 2 meltaguns) and call them the marine scout patrol or some such...
Add it all together, and people won't know what the hell they're facing. If you keep the
SW equipment to flamers and meltaguns, you'll still be fluffy for sisters and fluffy for deathwatch.
You can always just go directly to a normal marine army list and add the
DW as an option to it, but then you'd have to bring a elite inquisitor.
All in all, it has alot of possibilities whichever way you want to do it and most people will not be sure what to do with it.
Just something to keep in mind,
DW and Zealots are illegal currently in the
GT tourneys. Hopefully someone will change that.