The raws exist as a slider currently (setting stress vulnerability, for example), and I'm not sure what the myth sliders will end up looking like; does what the "violence level" slider become enter into this? Possible. For now, I'd just like to provide some non-broken texture and occasional problem dwarves, and heavy issues for those experimental forts where people intentionally mistreat their dwarves as a kind of baseline. With the next release, we should be a bit closer, though there's a possibility that people might be too happy again if they process their emotions too fast... it's hard to run sufficient long-enough tests from my position. Seems okay so far. I certainly don't expect that everyone will be satisfied, but the base stress vulnerability will always be moddable.
Since stress is already on a slider, and people seemingly aren't going to agree on how much stress a dwarf should be able to take, are you considering opening the slider for stress threshold to the players? Obviously the thought trains have to be fixed, but after that, it would be nice to get more direct control over how stressed your dwarves can get, according to your own personal preference.
Firstly, this is a suggestion rather than a question about the plans, so it fits better in the suggestion forum (which Toady reads, as far as I understand).
Secondly, Toady mentions that the raws exist as a slider: i.e. that you're free to adjust the raw contents to your liking, not that there's a single number you'd just adjust. Presumably, Toady's balancing aims at making what's in the raws in vanilla DF to meet the goals described. Since save game compatibility is a high priority, raw numbers are probably not going to be adjusted in the vanilla version without a very good reason, so the adjustments will be done internally.
You can make some kind of script to go through the raws to change them according to your principles (and if you think it's reasonably useful to others you can post it as a third party utility).
Question about messengers: When they are out on theyr mission and encounter hostiles (Armies, undead, titans, nighcreatures) will they report those? I imagine theyr coding will them make them walk by but on the returntrip (given that they are alive) will they tell you the news?
I suspect they won't encounter hostiles initially, but just magically pass through them, as messengers have the same issue as raiding parties, namely the need for coding logic for encounters. For messengers it would at least mean getting caught (and interrogated, or just tortured for fun...), killed, or passing unnoticed. It's less unbalanced to slip through as a ghost without issues than to slip through as an untouchable ghost that then reports back on what's seen.
When fleshed out I expect all sorts of trouble for messengers, as well as reports about the troubles managed (rumors or others would have to tell about the troubles that were not managed).