Here is an example of a really simple one (line follower):http://playwithrobots.com/simple-line-follower-robot/
You really have two sensible choices, but I fear that you will choose neither, and it may not work out as planned.
(1)...Go really simple, and stick with simple. Seriously consider ditching pure digital, and have it much more analogue, if it helps. See how others have solved this problem.
(2)...Go with a Microcontroller. It allows a simple, initial solution. With the capabilities, to continually refine and add functionality, to quickly improve it as necessary.
Attempting a complicated digital gate logic solution, worries me, as it would be much harder to change, and would have all sorts of technical limitations and issues. Digital logic is mainly 0's and 1's, but you really need more complexity to the signals (which Microcontrollers easily can do), and need to make much more complicated decisions, than seeing if something is either 0 or 1.
Great things can be done, purely digitally. But it can take immense skill, experience and education (possibly self taught), to achieve things like that.
Otherwise, FPGA's would be considered really simple.
If you really MUST have a complicated digital logic design. Perhaps consider a different type of project, better suited to a pure digital logic solution. Such as a frequency counter or digital stepper motor controller (possibly with Microstepping) or something.