EDIT: I take it when you say caster you meant DPS? If you meant a healer only, you may disregard this post.

As far as PvE magDK DPS goes, not much flexibility. I run with; 5 julianos, 2 valkyns/2 kena (situational) + 2x maelstrom destro + 3 x will power --> all max mag or spell power. Crit chance mundus.

Bar 1: Engulfing Flame| Burning embers | Whip | Ele blockade | magelight | ult --> meteor
bar 2: Eruption | rearming trap | elemental ring | Flames of OB / molten arm (depending on whether i want to use pots)| magelight | ult : standard

Opening rotation with ult : (la = light attack), Open with full heavy after using molten (if using molten), if not just pot it up then;

Eruption > la > rearming > la > standard > la > elemental ring > la > fob > barswap
Engulfing > la > Burning E> la > ele blockade> la
whip + medium weave > Repeat this row while keeping blockade up until engulfing runs out
repeat from eruption. ( for when u dont have ult, just skip standard)

As for PvP mag DK,

I run 5 black rose, 3 will power and 2 mal , with my same PvE staves (or snb/resto depending on situation) (sad but yeah it works)

For pvp im basically a tanky support while group kills. Just use talons, eruption, proxy etc etc with batswarm and mist for re positioning. Black rose coupled with mist and batswarm and malebuth is pretty darn OP on dks lol.