I'm currently working on a 2.5D space shooter and there will be a survival mode in the game. I'd like a few suggestions on how to tackle the enemy generation problem. This is not procedural generation at all, it's more like a sequence generation. The characteristics the system needs to have are:
- Increasing difficulty based on time
- Ability to use enemy patterns as input and not only single enemies
- It has to be beatable, it's quite easy to make a wall of thousand of enemies :)
I'm not looking for a fully-optimized, ready to use algorithm, just some suggestions or links to papers or articles. I have found many resources on procedural generation, but nothing that can be applied to this game genre. The game is 2.5D but the level will be generated as it was 2D, in addition it's not omnidirectional, it will scroll in only one way (this should make the task easier).