Sonic blast is very good on defenders. They have lower damage starting out, but superior debuff numbers bring it up again. It also has some nice controls to give space for setup, if need be.
I went with AR for concept
/Sonic /Rad and /Dark would all be reasonable choices with good AOE to leverage the acid mortar with and useful secomndary effects
/Archery will be the best for AOE, but its best tool (rain of arrows) in itself takes quite a while to go from initiation to damage, which coupled with trap set-up could be a bit frustrating.
@Catwhoorg "Rule of Three - Finale" Arc# 1984
@Mr Falkland Islands"A Nation Goes Rogue" Arc# 2369 "Toasters and Pop Tarts" Arc#116617
i've found that it helps offset the lackluster damage of the /elec set nicely, besides, it's really easy to fit concept with electricity and gizmos 
haven't tried it with anything else yet, but it works well with /elec...
Oh yeah, that was the time that girl got her whatchamacallit stuck in that guys dooblickitz and then what his name did that thing with the lizards and it cleared right up.
screw your joke, i want "FREEM"
Got my traps/sonic to 50 today and its been a fun ride.
Traps should work with all secondaries. I´m tempted to try all of them myself but i´m already short on influence since i´m gonna kit out both builds on my traps/sonic for different purposes.
Assault Rifle and Archery are good thematic links to Traps and both provide good AoE damage (especially so in the late game). If you don't mind the redraw, these are good sets.
Energy's Knockback is often maligned, but it's actually a really good case of mitigation. Traps doesn't need a whole lot of that, and the AoE KB may make keeping foes in the the debuff fields a little more difficult.
Ice brings in some good single target damage, but also stacks up some Slows and has Holds that can stack with PGT. Ice Storm also works well with Traps.
Electric is so-so damage wise, but adds in another debuff to the mix, plus its AoEs play well with Traps' PBAoE nature. Tesla Cage can also be used to stack up Holds with PGT.
Radiation, Sonic and Dark are the debuffing kings for -DEF, -RES and -ToHit, respectively. Rad and Sonic both stack up well with Acid Mortar, whereas Dark plays nice with Seekers and FFG. While Rad may be rather short on mitigation, Sonic has that in spades. Might not be needed since Traps has lots of that, but Sirens+Trip Mine is a fun little trick. Dark comes with its own Heal if you're into that.
Psychic's -Rech can play havoc with anything higher than a Boss, especially since it can stack up with Web Grenades' -Rech as well (an astounding 62.5% in just Webnade). Lots of mezzing to stack with Traps' own and Trip Mines will definitely help for when you face Psi resistant foes.
Personally, I went Traps/Electric as well, but really, anything goes with Traps.
Head of TRICK, the all Trick Arrow and Traps SG
Part of the Repeat Offenders
Still waiting for his Official BackAlleyBrawler No-Prize
Ok, so, having plenty of experience and an immeasurable amount of fun from Traps I thought I'd build a Defender using it rather than another Mastermind. I got that far, until I hit the wall: Uhhh, secondary blast set to go with Traps...? So, I need suggestions of any kind (concept does not matter at all) that would be helpful in guiding me to choose my blast set for my Traps Defender, thanks.