marketing campaign

What are some key elements to consider when designing a logo for an online marketing campaign?

i think memorable is the most important part, you would want your logo to be easily recognised and stay on top of their mind. Then you have to consider the simplicity because complicated or overly clustered logo are less likely to be remembered or understood at least from my experience. Your logo and campaign's purposes or messages have to be in sync. They should accurately represent the campaign.
Try to see it from your target audience's perspective.

Simplicity, color should complement the brand and should represent the brand