Tower defense games you have a pretty know set of enemies following a known path, etc. I wouldn't want that at all, in fact I would want to be further away from that than we are now.
The two problems I have with biters, is firstly every biter attack is basically the same mix, with just a simple global evolution scaling, which is the same every game, and secondly what they will attack and how much is entirely predictable.
I can build the same wall and turret setup at any base in any Factorio game and the biters wont beat me... Only real difference is the number of turrets I build due to evolution and pollution. I think to actually really loose a base outside the start of the game id need a monumental screw up like a pure laser defense and a total black out I cant fix quickly. Very late game when biters have lots and lots of bases that are fairly difficult to clear, its more tedious than anything to deal with.
A better variety of biters and defenses, with different attacks and resistances, if the defenses had better AI (so not just "shoot the closest thing"), and if such biter varieties were regional thus would be great, because copy-pasting the same defenses would not work. On the biter base front, rather than again vast numbers of identical thing to tediously clear, Id rather they didn't build new bases away from the player (leaving the late game explored map open nearly as much as early game), but a variety of different bases, such as stronghold bases. Few in number, but requiring a significant siege to destroy if you want them out of the way (with maybe going round and not poking being the safer option
![Smile :)](./images/smilies/icon_e_smile.gif)
).
Similarly, if the biters were to better probe for and exploit weaknesses would be a nice change. Things like occasional airborne attacks from over the water, biters tunneling deep into a base that neglected to lay down concrete or deploy other counter measures, biters occasionally attacking (although maybe with a 10 minute warning) critical non-polluting infrastructure (e.g. solar farms, and long distance power lines, but probably not rails without a practical way to avoid the problem), or massive attacks launched by previously mentioned stronghold bases (again with notice, and something the player will need more than just turrets to fight off, especially on co-op MP servers). Just general stuff that makes it less a case of building a standard turret wall then forgetting about it for the rest of the game (or until I abandon the outpost, to build an identical one on the next ore patch).