we already know that certain scenarios are planned to be a basic part of the game. Though it seems that you want more control to create your own scenarios. It might be interesting to be able to set up embark scenarios in a sort of combo between the raws and advanced world gen. Say like:
set immigration frequency: [this would allow you to determine how often immigrants come]
set immigrant number [the number could be random within a range defined here]
immigrant type:criminal [possibly defined as a specific class in conjunction with the crime and punishment arc, other ways to define it would be based off of profession, noble status, etc]
immigrant proportions [the ratio of this type of immigrant in relation to other types per migration wave with the total being set in the above example]
immigrant status: as a prisoner [ie in a cage or whatever],guarded, etc. .
of course this, I apologise, a very rough example. Other things that could be considered is when this sort of thing is an option to be chosen ingame, say you cant build a military fort unless you're close to an enemy[assuming that proximity will be a bigger issue once the army arc is completed], or a trade station if you're not near a trade route such as a road [once the trade system is expanded].
Then there'd need to be flavour text to make this look at least semiproffesional. And if diplomatic and other out of fort features are expanded on, there'd need to be ways to customise these new features to fit what you are trying to do. Ssay a military fort being able to ask for reinforcements or ressupply or whatever.