If you like mandrakes, then definitely get some mandrakes. The models are very cool and I've heard stories of their success.
You can never go wrong with more warriors and raiders. Something like 2 more boxes of warriors and one more raider, and maybe a venom or 2. Finally, a razorwing is an excellent choice for any list as they are just about our only anti air option.
Here is where it gets interesting. Trueborn with blasters, a court of the archon with sslyths and medusaes, grotesques (possibly in the grotesquerie formation), Scourges, Ravagers, Dark Artisan formation, reavers with caltrops, heatlances or blasters, incubi, or even a corpsetheif claw formation.
Most lists I've seen (including my own) incorporate a variety of the above units to support their core of mounted warriors. Personally, I love the dark artisan as it is a very tough unit that comes in wherever I want towards the middle of the game. Just make sure your units can keep up with the group; in transports, on bikes, arrive via webway, etc. General advice would be to double up on most choices since we die very very easy.
Finally, you'll need to pick an hq. I'm partial to the lone Lhamean in a venom because its cheap, though probably not the best fluffy option. A blaster/wwp archon goes very nicely with trueborn, a wych archite mixes very well with a grots, and a killy archon+court is a fluffy and powerful option. Any coven formations will require an additional haemy, who ain't cheap but can be given some fun toys to compliment his unit.