Shmup engine

Right now I'm building my Shmup engine, and it's a lot to think about. In the first couple of weeks of development I will definitely focus on the core functions. It's pretty important that it has good design from the beginning. I don't want myself in a position where I forgot a feature I wanted in the final game, and then realize it's impossible with the current design. Here are some core functionality I'm currently working on:

• Enemy movement with smooth paths, waypoints and formations
• Player movement, this game will feature 360 degree movement (only in 2D)
• Adding/removing entities in the game world and collision detection
  
• Flight path and movement for a stage
  
• Triggers/actions/behaviors, when will enemies attack in formations?
  
• How will the Shmup designer work, what will you be able to modify?
• Configuration in XML (stages, paths, waypoints, formations, triggers, behaviors, enemies, weapons)
• How will upgrades/powerups of weapons and player work?

This surely isn't all, but I must have something to work with.