A rather simple way to add longevity to invasion popularity is to add rotating random mob types.
For example, an invasion may be corrupted (normal) or it could be Angry Earth, Lost or Ancients.
The invasion should tell us before it starts what mob type it is so we can prepare.
By doing this, it will create a huge mixup of players, weapons and play styles.
Right now, invasions are dominated by bow because corruption. But invasions may see more fire staffs for Angry Earth. Or hammers for Ancients.