Secondary professions for a Dervish

Dervish Warrior
Armor-boosting Tactics skills make good additions for squishy Dervishes. "Victory is Mine!" works well with condition-causing Derwish skills.

Dervish Ranger
A Dervish/Ranger could use Apply Poison to dish out poison in an area of effect.

Dervish Elementalist
Dervish/Elementalist could have a good use of Conjure Lightning/Flame/Frost while having a scythe with the same element. This would give a not bad extra damage up to 3 adjacent foes.

Dervish Mesmer
A Dervish/Mesmer could use Illusionary Weaponry and cause armor ignoring AoE damage.

For farming or mass AoE damage, another good combination is Signet of Midnight (which also blinds the Dervish) coupled with Epidemic for spreading blindness. Using Sand Shards while blind makes way for repeated, heavy damage. Since Signet of Midnight works for the same time it recharges (15 seconds), one could technically farm forever, with only Faithful Intervention or Watchful Intervention for healing. Whirling Charge can speed up this process as well.

Dervish Monk
Probably this could resolve in a nice combination of Mysticim combined with Protection spells to get massive amount of energy back from your cheap protective enchantments. Contemplation of Purity could quicken this, and since many Dervish enchantments give a Dervish even more health upon ending, incredible spike heals and conditon removal is possible. You can also use skills such as Vigorous Spirit in conjuction with any Scythe Mastery skill- Dervish Scythe Mastery skills hit up to 3 adjacent opponents, so as you would get massive amounts of health from Vigorous Spirit. This could be very effective in farming.

Dervish Necromancer
Like any melee character, the Derwish could use Plague Touch. Barbs increases the damage dealt to his current target.

Dervish Assassin
The shadowstep abilities, knockdowns and conditions like the Deadly Arts make valuable additions to any melee-focused Derwish, especially in PvP.