What he means is less damage, more utility. Swain just does constant damage; a real poison mage would go something like:
Passive: Toxic Venom
As
hits enemies with spells, they are debuffed as follows, once every second:
1-10 stacks: 2.5% slow per stack
11-20 stacks: 2.5% slow per stack, -1.5 AR/MR per stack after 10 stacks
21-30 stacks: (Above), Enemy vision reduced by 15 per stack after 20 stacks
31-50 stacks: (Above), Enemy loses .2% max HP as magic damage per second per stack after 30 stacks
Therefore, if you are Dr. Mundo being healed/shielded by all 4 other teammates against Cass, you would be at:
10 sec in: 25% slow
20 sec in: 50% slow, -15 AR/MR
30 sec in: 75% slow, -30 AR/MR, -150 vision range
40 sec in: 100% slow, -45 AR/MR, -300 vision range, losing 2% max HP per second of being poisoned (liandri's)
50 sec in: 125% slow (functionally useless, but whateves), -60 AR/MR, -450 vision range (about half), losing 4% max HP per second, why are you still alive at this point.
If an enemy becomes un-poisioned, Cass has a grace period of 3 seconds to re-apply before stacks disappear.
So, an ideal poision mage is not "dee oh tee", it is debuffs applied over time, slowly becoming deadly to the hardiest warriors. Of course, Cass's damage would suffer rom having this passive, as she is no longer a sustained damage dealer but a long, kitey, drawn-out battle type of champ. Possibly rework her E into a long, this skillshot, a la Nid spear, to harass from far away and take advantage of her grace period.