You aren't going to be able to build a super-fast CPU on an FPGA but there are plenty of other tasks you could give it. You can build a processor for analyze multiple sensors, looking for specific events that you could then feed to the uC or setup a DSP pipeline to filter the sensor data to look for patterns. The nice thing about an FPGA is that there is no interruption when it's running. The uC might be trying to feed data over wifi to the controller station and polling sensors/analyzing sensor data is going to bog things down really quick. Another task for an FPGA would be to act as a switch for a network within the robot. It would sit between all of the sensors/actuators and an ethernet-enabled uC located elsewhere in the robot. The FPGA would read command packets from the uC and translate tasks like "read temperature sensor #5" or "turn 15 degrees" into specific impulses on the pins while also turning sensor and feedback data into packets to send back to the uC. You could even run video feeds from cameras back to the uC over gigabit if the FPGA supports it. This way you can run a cheap, small uC to control things from a higher level while the i/o-plentiful FPGA does all the heavy lifting at 100Mb/s speeds.