One of the factors in determining the strength of the base attacks is the size/power of your base. Adding to the complexity will increase the number and type of drones sent in a wave, and adding multiple bases (defence towers, storage/crafting facilities) will increase the number of attacks as each base will have their own independent timer.

Thanks for the anwer and Info. I completely forgot that a second Base or for example a landing Pad with a core would have its own timer. But still, do you or someone else know what game file needs to be changed to increase the difficulty? And in what way? I will also post the question in the general Discussion Forum. Should have done that to begin with.

If you are playing co-op, you should be able to adjust the difficulty settings of a save file before you launch. You can adjust the overall damage/HP of enemies in general, the number of drones planet wide, and the frequency of base attacks. If you are playing RE2 it is recommended to launch any new games with default settings and make any changes before loading a save.

edit - if you're using a server, I don't recall the files/locations enough to offer directions

Also, I would guess its possible to use the console for comands list type 'help baseattack'
it gives you a list of ways to manipulate and start base attacks