Stacks

The stacking mechanic is too powerful to be in an easy kit, and its inclusion actually prevents both Nasus and Veigar from getting meaningful gameplay changes. But this isn’t specifically about how they perform, rather how a stacking mechanic should be handled. And the short of it is: stacking should be relatively difficult, not easy. Riot’s actually doubled down on making the mechanic too easy. Rather than even choosing your poking and/or fights properly you can simply refuse to fight until you’re strong enough from only farming— and both champions have the means to do so.

Other differences between the games notwithstanding, HoTS handles stacking much better. The rewarded strength is generally on par with the difficulty of the conditions. Genji, for example, gets a slight damage boost only after he hits enemies 25 times with his shuriken. Kael’thas actually takes it a tad too far in the other direction, having a very high requirement on a difficult to land ability AND losing his progress in death. A happy medium between Genji’s easier-but-pretty weak and Kael’thas’s too-difficult-to-do-but-very-powerful would be perfect. Cho's on the right track with his having such a long cooldown and a cap to stacks from minions.

The other aspect that makes Nasus and Veigar poor is how the damage increase is directly in line with the stacks rather than set at specific milestones. So it rewards mediocrity by still giving us the bonus 30 damage from 30 stacks, even if we should’ve had 50 at that point. We aren’t properly hindered for stacking poorly. And unfortunately this also forces their kits to be entirely revolved around the stacking. Rather than being an optional reward, it’s just a necessity for performing on par mid to late game. (And the ease of stacking from farm instead of battle renders this impossible to change with the current iterations of the abilities)

I don’t really have a solution for the problems presented by current stacking champions, but if more are introduced they need to have more difficult conditions. And if Veigar/Nasus are reworked, they need to either be made more difficult or have their stacking changed heavily to enable easier adjustment of their whole kits. I know there are some who would argue that the stacking is core to the champions, but a core that is unhealthy to the whole is a core that needs to be changed.