Widespread diggers? Eh, who said anything about 'widespread' diggers?
I meant as every siege containing at least 1 digger unit, changed by race constraints. Goblin sappers, orcish marauders, heck, kobold engineers finding a way into your fortress is fine. Doing it every siege, every time, is not.
Unless i'm taking what you said out of context here, and misunderstood your use of the word. All we would need is diggers to be programmed. The AI doesnt exist now, but it needs to be added for this to be in - this isn't something that should be just slapped into the game. "Due to AI constraints". Well you fix the AI to do what you want to. Like any other feature Toady adds, there is more than 1 piece to it going in. Better AI would be a part of that. Mobs would need to dig only a certain amount, and not go partying with a pick-axe across the entire map.
Problem is Toady's approach to pathfinding, I'm afraid. The AI constraint phrase I mentioned is that there'll have to be a better way to pathfind for creatures - the basic sieger ai just calculates given DIRECT paths to your dwarves, and closed doors can prevent this. And with randomized worlds, forts and what have you, designing a working algorithm for pathfinding to also include digging may be too much. It is possible, but it won't be in a small time scale. There are other arcs coming in and whatnot. Then there's the possibility to exploit any given pathfinding algorithm. There should be upper limits (and lower limits) to digging, 0-6 diggers per siege, 12-48 tiles per tunneler, whatnot.
Starting difficulty? Starting difficulty varies. And last I checked: Losing is Fun! How does this affect starting difficulty much, if at all? You won't get sieged for a while unless you are close to a hostile civ. Presumably you wouldn't be getting tunneling 100% of the time, either.
Well, Losing is Fun, unless you're bound to lose without even having an option to change the outcome. I mean, you can isolate dwarves against tantrum spirals, you CAN always savescum and design some traps, you can change your plans, you can airdrown the carp etc etc. Things should remain random, but without any option to see your effort's outcome, losing becomes un-fun very fast.
Difficulty depends on where you start. "Starting difficulty higher than intended." How is that possible? If you don't want to be sieged early, don't start a fortress really close to hostile civilizations. You set your own difficulty by making the appropriate choices, and then counter-acting them with defenses and alternative plans. If you want to, then take the proper precautions. Just like you wouldn't build a fort really close to a demon-owned goblin fort and expect to not build a gate and fortifications somewhat early on. If you don't want a "higher starting difficulty", you build away from it, and take the proper precautions a little more liesurely.
This "causing" difficulty to be too high too early on is a misconception. Location affects difficulty far more than this. Location dictates how soon you might have sieges. Sieges would dictate a potential for tunneling. While this may increase difficulty for those seeking more difficult games, it does not affect difficulty in any other way early on. It cannot. Not unless you plan on having a safe location and pissing off some elves or humans really early for laughs. But then that's your own fault.
You can't 'build away' from tunneling siegers, though. That's the problem. There should be either raw-controllable tunneler options, or no tunnelers, because of the swiss-cheese problem and dwarves having no relatively 'natural' way to rebuild a scenery. There's also the fact that all of these things are a personal matter - I have given gobbos higher damblocks, for example. I like them a bit resiliant, and more aggressive. But I don't want orcs. You, for example, might be so sick from the lack of sieges, you installed orc mod and gave them absurd damblocks.
All I say is -so far- Dwarves are
the diggers of this universe as default. And it'd be absurd for non-dwarves to dig as proficiently as dwarves (AI constraint) or fast as dwarves (Difficulty). They should not cause random cave-ins (AI and Swiss-cheese constraints), or they should be re-routable, preventable, somehow.