The future of the antidepressants lies in chemicals that increase serotonin at the 5-HT1 receptor (lowering depression) without affecting the other receptors. Drugs like mirtazipine and nefazodone attempt to do this by blocking certain 5-HT receptors (mainly the 5-HT2a receptor, which lowers anxiety), and as a result make serotonin levels higher and more available at the 5-HT1 receptor. That's the reason these 2 drugs in particular don't have the nasty side-effects of SSRI's.
The future of anti-depressants likely lies in the ability to find a drug that increases activation at the 5-HT1 receptors in certain parts of the brain, while leaving the rest of the brain and receptors untouched.