They either need to refresh the game by adding a new gamemode or just diversify the waves of monsters.
It gets annoying to play EXACTLY same wave progression.
You know, 80% clots, 10% bloats, 10% gorefasts during 1st wave.
50% clots, 20% crawlers, 15% gorefasts, 10% bloats, 5% stalkers during the 2nd wave..
Then sirens, Scrakes and finally fleshpounds appear.
It is fun at first, but since there's only 1 playing mode, it gets REALLY repetitive and dull, since you expect and KNOW what will come at which point of the wave.
Simply varying out the units BUT maintaining similar wave power level would refresh the game QUITE a bit and wouldn't be hard to do at all.