- Art (as that takes the longest time to get 'just right'), both for external & internal (NPC/station) models. Combine with below for suitable 'hidden stash' 1P, 2P & 3P placement spots as necessary
- AI, as it's the most often player encounter
- Scripting (tools/additions/modifications), as this allows modders to fine-tune their creations better. Possibly combined with AI above..
- Mapping/routing/pathfinding (possibly combined with 2 & 3 above)
This is not meant as a generic list, simply a guideline/roadmap. Anyone care to add things I've missed..?