The ability to add conditional to invasion groups, so that a given situation can change what mobs are spawned in an invasion.
For example:
Being able to spawn a group only if the player is submerged below 10 blocks of water in an ocean biome.

This would allow invasion developers to tailor the invasion based on the players location. And in the case of the example actually make it feasible to have aquatic invasion waves.

My attitude is this would be impractical as described, and implemented as is, it would promote an overly complicated for a configuration.

I do see a future enhancement need for:

1. Having an invasion condition of the biome the player is in.
2. enemies that are allowed to spawn in water.
3. API for dynamically creating an invasion at runtime. That way complex rules (such as the one you described or more) are not constrained by the config. That way, if you want an invasion to be an option, or a condition, there's a manner for people to hook it in, but without needing to fork and enhance Onslaught^tm