Some more ideas:
FD:
- WW1 tank design challenge
- Interwar tank challenge
- Technology demonstrator challenge
- Supersonic Transport (SST) challenge
- Light Tank challenge (era up to debate)
- Armored Car challenge
- Tank Destroyer challenge (any era)
- "Prototype" challenge (one-off or limited production air/ground designs)
- Airborne Aircraft Carrier + Parasite Fighter challenge; (resuggesting)
- Helicopter challenge
SB:
- World War 1 Cruiser challenge (Light or Armored); personally leaning to the former
- Interwar Destroyer challenge
- Export ship design challenge for a foreign navy (open to debate on timeframe/specifications)
- Destroyer Escort challenge
- WW1/WW2 Destroyer challenge
- 1st "radical" ship design challenge (a-la Fisher's HMS Nonsuch, HMS Dreadnought, HMS Incomparable); resuggesting
Also, would be open to doing challenges in the new "Charscale"