I think the real problem with stun is when it is used on a solo or, to a lesser extent, on an elite. After all, stunning one standard monster out of five still leaves the other four able to act. Hence, I'm considering taking the following approach for stun, which gives the stunned creature the option to do nothing (which is the same as current), or to pay a price in order to do something:
A stunned creature is dazed. In addition, if it takes any action, it takes damage equal to 10 + twice its level.
Since solos and elites have more hit points, it is proportionately less painful for them to act while stunned. However, if they choose to do so, the PCs still get an advantage (more damage) for having stunned the monster.