One of the advantages of the End Times list is that we don't need a lot of other models if we have Malekith
Your existing choices are a good start.
And your idea of Dark Riders/Warlocks an ideal expansion. I'd go for 10 of each at least.
- three more bolt throwers
- 10 Waywatchers (or Shades)
- a Pegasus character
...and you should be on your way to a mobile, elite force Disclaimer:
Daeron was posting while I was posting. +1 to his comments.
Playtest with proxies first to ensure you enjoy the playstyle and troop choices