When the bugs that gather up uranium for the reptilians are added into the game, you can make it so that the uranium will slowly kill them off, because I don't expect the reptilians to give them any protection (since they are slaves).
This will make uranium gathering a little bit more challanging (because from the start it is a precious resource) but it will also make it more interesting since you have to make sure you don't let them die. This could be prevented by adding a new building that heals them when they are damaged.
The same can be done for the other types of bugs, but instead of radiation poisoning it will be exhaustion from too much work. The same building that I mentioned above could let them rest before they continue gathering (or perhapes another building).
Hope you like this idea.