For making base defenses more interesting or more rewarding there could be a reward for successful base defenses that allows you to build more structures of a specific type.
Changing the difficulty selection and initial buildlimits:
The three buttons for the difficulties could be exchanged against a button for each enemy faction… And the difficulty could be increased automatically.
Let’s say, that our first base starts with a buildlimit (walls, defensive structures,…) of 50% of what we have now.
Selecting enemy factions:
We may choose to start a base defense mission against soviets or fnix. If we start with one of these factions, it’s locked until we finished the final defense mission or fail. (this part of the idea could also depend on an other idea with moving frontlines… Or a level of influence of a faction in a region)
Each time we’re successful, we get a reward like it is now + get a bonus on the buildlimits. This could be a overall bonus by percentages for each category of structures, or a fixed bonus for a special category or structure.
At the same time the level of the base increases and at every (or every 2nd, 5th,…) level the difficulty increases, too.
Automatic difficult increase: (against fnix)
While you start your first assault against prototype runners and hunters, they could become more on further levels. Then they get a harvester, or a tank and then some more.
At a specific point (let’s for example say level 10) the prototype machines will be replaced by military machines, at first again just runners amd hunters and so on.
At level 20 the same happens with fnix machines, at level 30 with apocalypse machines and finally we get a reaper.
Remember, you get more and more upgrades for your base like described above.
But if you lose, your base will be reset.
Base defenses against soviets:
For soviets it could work the same way, including firebirds, but there is a final boss unit missing until now.