Would have to wait for the fortress starting scenarios improvement, but dwarves in a fortress that gets abandoned should return back to the parent civ and be available for future embarks.
When a fortress gets abandoned, take all the surviving dwarves and track them in a group linked to the parent civ. All their skills, stats, and equipment, plus any other owned items they can easily carry, stay with them.
If the fortress was abandoned in an ambush, siege, or any other sort of attack, each dwarf has a random chance of getting killed trying to escape. Injured dwarves have a dramatically reduced chance of surviving.
When preparing for embark you get a list of the survivors from previous fortresses and can choose them to replace any/all of the standard random starters. The embark cost is based on their skills, but at a decent discount (explained later). All their equipment stays with them at little or no cost to you.
Married dwarves may require their spouse to be in the embark group, or their spouse will be in one of the first 3 immigrant waves. Children follow their parents, you gets no say or warning.
Dwarves returned from abandoned fortresses have a semi-permanent unhappy thought stemming from the previous failure. They saw their last fortress fail and therefor aren't very optimistic about the new settlement. Especially if you reclaim their previously abandoned fortress, it already failed once, what's so different about this time around?
This scales with how happy they were on average in their previous fortress. If the scaled unhappy thought brings them below a certain base happy level, they get removed from the list, too emo to try again.
This is the reason for the embark cost discount, they may have greater than 5 in some skills but they're also a lot harder to keep happy.
This unhappy thought slowly decreases in severity over time, and certain events can decrease it quickly. Like killing off a siege, downing a FB, fortress wealth reaches certain thresholds.
Returned dwarves will also have a permanent attribute that causes them to complain to the leader/mayor/ect... more often. They think they know what went wrong last time, and they're going to be sure their leaders never forget.
Good idea, no?
Sorry if this is a common suggestion I've somehow missed.