First of all, I would try to take advantage of some of the formations in the Codex. The Blackspear Detachment is excellent, and although you do not have access to things like Objective Secured, as you would in a Combined Arms Detachment, the re-rolls and mobility you get could prove very useful.
Here's what I like about the list so far:
- Bikes. Twin-linked with special ammunition, these are awesome. Since you can take them in as little as units of 1 model, I would take advantage of that... I don't know what the sweet spot is, but perhaps 2 units of 2 in your case?
- Stalker pattern boltguns. These are tempting, and can be quite good, but keep in mind that they are still "heavy" weapons. Depending on how you deploy/bring on your models, you may be better off with regular boltguns at reduced range; especially if you're pairing them with other weapons of short -> 24" range (frag cannon, etc.)
- Watch Captain Artemis. What a cool model, and he's got that really awesome D bomb! Very fluffy, and probably the model that singlehandedly drove you to the army. You can make him work.
Ok, so now, how to adjust things...
First of all, I'd take the aforementioned detachment, or at least some formations. Artemis is your
HQ, you need one killteam formation and one aux now to fill it out.
I would take a Corvus Blackstar. It provides a lot of horde control, which you'll need in the event that your army is purely Deathwatch, because of your low model count. They excel at killing high profile targets, and hordes... but you have to choose and not mix squads.
Don't overlook the Deathwatch Shotgun either. It's stock profile allows for some nice AP4 double-tapping... this is really good out of a pod (or deepstriking) and pairs well with the frag cannon
IMO. They are also assault weapons, so you could pair up some melta bombs too to make that unit pretty versatile.
Also, a Venator Killteam is a good choice if you need to kill fast attack units (play against a lot of bikes, etc.) and with all bolters, it's a nice T5 special ammo unit. If you deepstrike it with this detachment, it's pretty cool.
Of course, the Furor Killteam is a staple, because you will ALWAYS fight troops... and Deathwatch are very good at killing most troops. My favorite Furor team make-up is one that ignores cover... so all templates. Shotguns, frag cannons, and a terminator with heavy flamer. It's brutal!
I would focus on some of the killteams, and trying to get more bodies and mobility on the table. Things like heavy thunderhammers, storm shields, etc. are all very expensive, and on an already small model count army must be taken in small batches, and on units with very specific reliable roles.
Try to think of some of the armies/units you may face... Scat Bikes for sure, Riptide, summoned demons, marines in rhinos/pods.. you have most of the tools to deal with a lot of these things, but you need to do it by using each unit to its fullest potential and optimizing their loadout accordingly. My general rule of thumb is... "How are they geared-up in the codex pictures/examples? Ok, that's exactly how I SHOULD NOT build them"

haha I mean, when's the last time you saw all frag cannons and shotguns in a pod in the codex... all you see is power swords and plasma pistols
lol!
Hope some of that helps!