Seasonal spikes in queries and search traffic driven by
offline marketing efforts will warrant having an extra "slush fund" for paid search -- because you can't forecast every time that clicks will ramp up.
As the engines constantly
improve relevancy, average CTRs will increase -- and if you've done your work, so will conversions. Average CPCs may also rise as more competitors get into the market and test their overall PPC
strategy (including landing pages and ad copy), and drive up the max bids for everyone involved.
Lastly, mobile, video and social media ads will become more widespread within paid search, and those formats naturally cost more than text ads.