Been having fun with this mech-based warlock that I kinda put together on a whim:
Played 8 games with it and lost 2. (went from R14 (??) to R12 5stars)
Fel Cannon:
>wrecks Paladins/Zoo/Face Hunters quite a bit since it gets a free kill vs any 1-2 health minions and can be ramped up with Mechwarpers, for which paladins don't really have a 1-3 mana answer to besides a minion + Seal of Light or Coghammer. You can usually throw out 1-2 other mechs for the ramp value before they get to kill it off as well.
>Belchers either die with 1 hit (Dark Bomb/Attack + end-turn ping) or gets their slime deathrattle pinged to death for free, assuming it gets played alone vs the cannon.
Power Overwhelming:
>Great finisher, can also be combo'ed with Shadowflame for a full Boardclear vs Deathrattle Minions, or to set up next turn lethal along with a board clear.
Hobgoblin:
>A gimmicky one-of tech card for your 6x 1 attack minions (Annoy-o-tron/Micro Machine/Demolisher). It does hinder your Fel Cannons so you generally don't want to play it until you get 1+ 1 attack minion to combo with.
>Still, a 4/4 Micro Machine (that can still grow), 3/4 Annoy-o-Tron and 3/6 Demolisher makes for really crazy board presence.
Arcane Nullifier X-21:
>Really bad against priest for obvious reasons (well, just Cabal Shadow Priest and Vol'jin). Pretty good against most other classes however:
-Mill Druid can't Naturalize/Swipe/Wrath
-Rogues can't Sap/Backstack/Evicerate
-Mages can't Fireball/Polymorph, and they can't Ping after Flamestrike
-Warlocks can't Siphon Soul/Dark Bomb/Implosion
-Hunters can't Hunter's Mark/Kill Command
-Shaman can't use any of their spells, even the famed Earth Shock that can deal with Twilight Drakes/Divine Shield minions.
-Warriors can't Shield Slam/Execute
-5HP means it doesn't auto die to Drakes/Truesilver/first charge of Death's Bite)
>Obvious Mech Synergy
>Only problem: Can't be used for PO or SF.
Dr. Boom:
>Probably the most-used Legendary in GvG, but still a really good one in this deck. While he technically hinders Fel Cannon's end-turn effect, you probably won't see him survive for one turn anyway, considering all the BGH and removals being run these days.
>Can be combo'ed with Hellfire the following turn (or same turn if you still have the coin) for a finisher, wiping most boards (aside from Handlock) while allowing you to potentially push for Leathal.
>The Bombs can also be used for the PO+SF combo for board wipe and are Mechs (synergy with Fel Cannon)
If you don't like the Hobgoblin/Demolisher stuff, you can probably take them out and replace them with something like 2x Spider Tank and 1x Mechanical Yeti/Enhance-o Mechano. Even KT could work if you think you can consistently set up a board for him.
Disclaimer:
Probably not legend-worthy, but still very fun.