There's not really a "pure" dot build. Even with a thief build, I would always pick up whirlwind.
That being said, for thief it is good to have at least 1 reliable way of dot due to the thief perk, twist the knife.
Additionally, eye for weakness is incredible with the grease spell.
Incidentally, my kitsune spellblade with Blue Flame Blade, Venom Strike, and Offhand Kunai does insane dot damage that stacks up really well with eye for weakness.