Every certain period of time (can be linked to the same intervals as difficulty scaling) an upgrade pack can be dropped somewhere on the map. It can upgrade one of the following:
max hp increase
heal
damage upgrade
attack speed increase (damaging, corruption)
move speed increase
corruption range increase
Single upgrade pack can randomize 2/3 option from the ones above and player will have to choose one of them. Another idea is to force player to pick upgrade in X seconds or it disappears. It will create a pressure on player, but it might be annoying depending on circumstances - will need some testing.
Every certain period of time (can be linked to the same intervals as difficulty scaling) an upgrade pack can be dropped somewhere on the map. It can upgrade one of the following:
Single upgrade pack can randomize 2/3 option from the ones above and player will have to choose one of them. Another idea is to force player to pick upgrade in X seconds or it disappears. It will create a pressure on player, but it might be annoying depending on circumstances - will need some testing.