Hearthlings can already detect a “damaged” building, in the case that say an ogre breaks the door down or some other piece of furniture gets destroyed. So clearly, the templates are already built to work that way, and if enemies are able to bust through walls then I don’t think it would be too far off for hearthlings to be able to automatically replace/repair those blocks; just like they do with furniture.
I agree, giving the player more busy-work is never a good thing; but it seems as though Radiant are already including the kind of tech they need in able for hearthlings to automatically repair such damage.
I reckon that would be really cool as a late-game feature – you build a wall, the enemies breach it, and then your soldiers hold the gap while workers or masons rush in to handle the repairs.
I’m also a big fan of @Aviex’s suggestion regarding ladders. I agree it’s the perfect “low level” threat – on their own, not an urgent worry; but if you ignore them then eventually you’ll have much bigger problems to contend with. Of course, they can also be turned to your advantage too… aside from the fact that the goblins are providing access to new areas; you could always use the ladders as choke-points or place traps and turrets where you know the enemies will be coming up/down.
Eventually I’d also like to see flying and swimming enemies; and maybe even “tunneling” ones (I think that having them dig up the terrain would be a bad idea, but maybe they can just move through dirt blocks with an appropriate animation?) More than just having enemies which can break through walls, I eventually want to see enemies which ignore that kind of defence entirely; and require different strategies to deal with their attacks. I mean, I’d go so far as to have spirit-type enemies (ghosts, shades, etc.) which both fly and ignore terrain; so the only way to defend against them is either the appropriate magic-based attacks to defeat them, or else to build some kind of magic wards to keep them at bay.
Naturally this is getting towards end-game content here; evil spirits which ignore your mundane defences wouldn’t come out until you’ve contstructed basically impenetrable defences anyway. As @Yosmo78 said originally, the idea is to make the player adapt to the enemies’ adaptations, so that there’s no single strategy to defeat all enemies. At the end-game levels, an invasion isn’t just a handful of zombies but a coordinated siege with a Giant Zombie to break down walls, some archer skeletons to help the zombies get into attack range, a Necromancer summoning reinforcements, and spirits hitting you from unexpected angles… as a player, you should be able to deal with all those things easily if they attack individually; but keeping on top of the different attacks is where strategy comes into it. You can’t just leave your anti-ghost wards out in the open, or they might get destroyed by the Giant Zombie or even the Skeleton Archers; and you can’t rely on your archers in a tower to do all the work because something will eventually make its way up to them…