I find it useful to envision as many future requirements as I can think of. That way the architecture can be designed with future upgrades in mind, even if these features are never actually implemented.

• Support for multiple display sizes and types
  • TFT LCD
  • Character STN LCD modules
  • Graphic STN LCD modules
  • eInk displays
  • OLED screens
  • Analog dial gauges (via PWM)
  • LED displays (bar graphs, 7 segment, matrix)
• Support for Android / iOS devices as additional graphical user interfaces and for internet access
  • Smartphones
  • Tablets
  • Smartwatches
• Synthetic Vision System (SVS)
• Moving Map View
  • Airports / landing strips
  • Airspaces
  • Current and predicted Weather
  • Waypoints and path planning
  • Other Airplanes in the area
    • Via Automatic Dependent Surveillance - Broadcast (ADS-B)
    • Via Low-Cost ISM-Band Transceiver (for friends in your group)
• Engine Monitoring (multiple engines)
  • Revolution Counter (RPM)
  • Hour Meter (Hobbs)
  • Maintenance / service timer
  • Exhaust Gas Temperatures (EGT)
  • Cylinder Head Temperatures (CHT)
  • Coolant Water Temperature (H20)
  • Oil Temperature
  • Oil Pressure
  • Manifold Pressure
  • Alternator Voltage
  • Alternator Current
• Fuel Management (multiple fuel systems)
  • Fuel Level
  • Fuel Pressure
  • Fuel Flow (Instantaneous, Average, Accumulated)
  • Fuel Range / Endurance
  • Fuel Pump Control
• Navigation Instruments
  • Heading
    • Magnetic Heading (compass)
    • True Heading (GPS based)
    • VHF Omni Directional Radio Range (VOR)
  • Speed
    • Air Speed (pitot tube)
    • Ground Speed (GPS based)
    • Vertical Speed Indicator (pressure differential)
    • Vertical Speed Indicator (GPS altitude differential)
    • Glide Ratio
  • Altitude
    • Barometric Altitude (user QNH setting)
    • Flight Level (QNH=1013.2 hPa)
    • True Altitude (GPS based)
    • Radar Altimeter
    • Height above runway (ultrasonic sensor)
  • Attitude and Heading Reference System (AHRS)
    • Artificial Horizon
    • Inclinometer
    • G-Force meter
    • Angle Of Attack (AOA) Indicator
    • Turn Coordinator
    • Turn and Slip Indicator
  • GPS Position Coordinates (multiple formats)
  • Distance covered
  • Flight Time
  • Reporting point helper
    • Distance and heading to nearest reporting point (e.g. 3 miles NE of XYZ) for radio position reports
  • Waypoints management
    • Route and waypoint editing
    • Course Deviation Indicator (CDI)
    • 3D Heading to next waypoint
    • Vertical Distance to next waypoint
    • Horizontal Distance to next waypoint
    • Time to next waypoint
    • Default waypoint: takeoff location
• Flight Data Logging (black box)
• Visual and audible warnings (navigation, thresholds exceeded, etc)
• Interface with other Avionics
  • Altitude encoder for Mode C Transponders
  • Transponder Remote Control
  • Intercom Remote Control
  • Air Band Transceiver (Radio) Control
• Pilot Notes / Checklists / Frequencies
  • Edit
  • View
  • Upload
  • Download
• Aircraft Status and Control
  • Battery Voltage
  • Battery Current (charge / discharge)
  • Flap position
  • Landing gear
  • Strobe light
  • Headlight
  • Heater control
  • Heat Vest control
• Stopwatch / Countdown
• Density Altitude
• Ambient Air Temperature
• Date / Time (from GPS)

This list is by far not complete, but should give a good starting point for the detailed design.