Hmm. I like this thread a lot better in the suggestions forum.
copx, I believe you mentioned at one point that you could only get up to about 50 dwarfs before you got bored with the pace of the game. While I realize this doesn't really respond to your suggestion, I do encourage you to get a little bit further - the fun for me really starts kicking in around 80-100 dwarfs.
Of course, that leaves your (legitimate) complaint that the game isn't very exciting before this point (and I guess you might also disagree with my assessment that it picks up a little after). While your description does seem a bit at odds with my experience - personally, I get regular ambushes of goblin archer squads when caravans arrive which are difficult to deal with - I do agree in principle. There are ways in which the game could be made more interesting and dynamic, in the sense of giving more of a challenge.
For example, giving goblins the ability to tunnel, while probably complicated from a coding / AI standpoint, would make seiges MUCH more interesting. It would give an incentive to set your fortress deeper into the mountain. You would have to think about the physical location of different things - keeping rooms in safer spots, for example. And imagine goblins tunneling while your own miners simultaneously try to cut off their tunnel to let your military flank them? Or dig out a big hole under them and collapse their tunnel into a chasm? (This would work better if goblins mined slowly). Furthermore, imagine that goblin tunnels are not visible until you stumble upon them, like the ambush groups themselves.
The principle argument I'm hearing from those who disagree is also legitimate: while the player can find ways to make the game harder or more challenging, such as choosing to not farm in the winter, or starting a fortress above the treeline in a mountainous area with no soil (not to mention zombies and undead), a player could not choose to make life easier for themselves if the structure of the game is made more difficult. Embarkation, as many see it, is analagous to picking "easy, medium, hard" from a difficulty menu. If goblins are made super-difficult, or farming is nerfed greatly (say, soil being overused leading to failed crops), players cannot choose to make the difficulty easier for themselves if they are finding the game frustratingly difficult. Short of modding, of course, which is another story I'm not going to go into.
However I think there is some sort of happy medium here. For example - one of the biggest challenges at the moment is when the economy kicks in with the arrival of the baron and tax collector. Suddenly, many dwarfs cannot afford to house or clothe themselves, because they do nothing (animal caretakers or cheesemakers bumming around) and thus make no money. Dwarfs get evicted all over the place and the player is usually faced with a huge housing crisis (at least I always am). This can have two effects.
One, if it's your kind of challenge, is to try and rework all of the job permissions to set up a fort where dwarfs are working more towards what is required, and getting things done. Also one might try to redesign housing. This can be very nitpicky, but has the added bonus of (often) causing fortress wealth growth, attracking more dwarfs and goblins and futher adding to the challenge.
Two, if the idea of messing around with all the job permissions makes you die a bit inside, is to go to the init file (as the init file is standing in for a "gameplay options" screen in-game, this isn't modding) and turn off room rent on the relevent line, setting the rent for all rooms to 1. I do this sometimes, say if my housing cuts through a magnetite layer and the room rent is astronomical. It's less of a challenge to build all new housing in rock areas while also redesigning the economy and production of the fortress, and more of a hassle.
To come full circle with this rambling post, further increases in challenge could follow this framework. Just as invasions can be turned off, the addition of tunneling, say, or more difficult / unforgiving farming systems, could be disabled in the ini file. This seems to me like it would keep everyone happy. There would be more challenge (and a more interesting game) for those who are bored, and people who are new to the game, don't enjoy the changes or would prefer to focus on building their 20 story crystal tower under the lake can turn it off in the init file.
I must say in the end though, I do find it hard to believe that there's no fortress site you can find that doesn't pose some sort of challenge. The possible sites vary hugely; I don't remember if you mentioned how many forts you've played, but playing many forts for some time rather than one fort for a very long time will give you a much better idea of the many different situations DF can throw at you. I'd been playing for a long time, but then I started a fortress in a region with zombie camels, and it was a completely different dynamic.
Nevertheless, your complaint is legitimate and I can see where it comes from. There have definitely been times when I've gotten bored of a fortress around the 50 dwarf mark. My suggestions are tunnelling for invaders, and degridation of soil. Both of these would make the game harder by making it more INTERESTING, which in the end is the important thing.