You can ask, but I doubt the agent will comply. Fifteen percent U.S. is standard, twenty for foreign, so I don't understand why you'd want an agent to lower his or her rate. What would be their incentive for doing so? Agents are in high demand, writers are not, so your request is a losing proposition. But like I said, you can try.
Other "stuff" in the contract might be considered, I suppose, depending on what it is. My agency's contract states they will represent everything I write, and I'm good with that, but not all writers would be if they want a different agent for nonfic or something. My contract is pretty boiler plate, really.