Aesthetic first[/B]. Would the build feature be totally free? Like placing every single props or buildings by hand and terraforming the map like a pure sandbox? Or would the build feature be in some way more restrictive? Allowing player to only build presets and upgrade major buildings?
Warband's Custom Settlements (and the follow-ups in other mods) did it by presets. It worked, if not without issues related to the restrictiveness of the engine.
Bannerlord is a spanking-new custom engine. Either way could work, even if hand-placing all the elements might not be something a lot of people would want to do due to perceived tediousness of the task. It's not like you couldn't combine stock settlement scenes with some kind of editor.
Economic: What about the player's evolution curve? How about the main map? For example if you create / add new villages. It will take some spaces and could destroy the balance of the whole game. How the lords will interact with?
Either allow this on currently barren parts of the map (and include restrictions on the peasants going to other towns, I guess), or, say, make it dependent on "village slot" unlocks through town's or castle's prosperity. This would also help with the "starvation through prosperity" issues, and general balancing out of town's prosperity potential - something that is strictly restricted right now by initial assigment of number of villages. A reason why Sturgian towns (with two villages to their title) will never match a lot of other towns in their potential. Allowing village slots unlocks at specific prosperity levels would at least address that imbalance - assuming the towns, and their villages, don't suffer from growth reversion due to constant warfare events.
In itself, it should not be a project the player can engage in on the get-go (unless cheats, I guess, but that's self-inflicted). Balance its accessability through demands of a lot of resources spent before even the first few huts pop up, and take it from there. It gives the player a non-warfare long-term project to dabble around (and drive their gameplay focus). Hell, you could actually pace it so that the generational gameplay that we already have but will rarely, if ever, see in play actually matters. So your son might inherit a developing village, and leave a fortress of a castle to their progeny (and make it modable so that people who want that to happen faster can do it).
Shouldn't be that difficult to code in lords to try to (depending on their personality traits, anyway) set aside enough wealth to start such projects on their own. Make "generous merciful" or whatnot lords more likely to start investing into their fief than "greedy devious" ones. Maybe add the possibility of a lord's senior (clan leader or faction leader) doing some general improvement of their own, even on others' fiefs.
The main problem is that Taleworlds decided that villages shouldn't be independent in Bannerlord (a decision I'll never stop complaining about, because "what were they thinking?!"). So even if you end up a faction leader of an independent kingdom, with tons of wealth to dump into such an expensive project, you'll never be able to start "growing" spanking brand new villages of your own as a way to attract minor lords. Technically could use this mechanics to get some new castles (in a long, long-term project), but unlike a village, it's not something you can base factional strength growth about - it will just take too damn long. This change also prevents (at least in current implementation) from "re-binding" villages to different castles/towns. Something that could have helped to shore up some of the inherently "poorer" towns at the expense of castles, if the player desired so, or wanted to prevent about-to-be-captured castle from taking away a much needed food/production source of their town. Not to mention making villages a viable diplomatic trade good when suing for peace, or even ransoming a faction leader.
I don't really see how this would be a problem in terms of balance. Either you make it "faraway nobody cares about until it hits some wealth level" possibility (so that player can genuinely start a real, independent kingdom through non-warfare means, which in itself would be nice now that we have a lot of non-warfare skills to build specific characters around), or it's just a piece that falls into similar slot as any other villages/castles/towns on the map.
It would be in fact a very damn cool feature but we should take into account how many issues it will raise and how to bring the build system working properly with the rest of the game.
We won't know if there would be any meaningful issues at all since it's something TW gave up on long ago, before the rest of the game was in any kind of semi-complete state (if it's now). Personally, I think the "balance" concerns are overblown - it'd have been a feature that wouldn't see constant occurrence, could have been balanced out by sheer time and resource investment tied to its creation, and at worst would simply make it easier for some location to get more prosperous (assuming nobody comes around raiding or ownership-flipping, which is a BIG assumption). On the other hand, aside from promoting a more peaceful playthroughs (though... not necessarily, either), it would provide a genuine long-term goal that even "vassal" players could engage in.
Like making settlements destructible could be starting point.
They kind of are. That's pretty much what happens when they get raided, even if the recovery time is pretty quick. If you mean complete removal of a settlement from the map... only if there is some form of dynamic settlement placement implemented, because otherwise I guarantee you the AI will end up with an empty map
All of this need to be clarified and if some modders want to do something about it later they would need to prepare a solid document about it
I have faith in people who will tinker with Bannerlord's code, because they already managed to do it with crummy Warband engine.
I'd much rather see at least the basis for it implemented by Talewords as part of the base game and engine, though. If only so that people who mod can focus on adding other things, because the way things seem to be headed right now they'll be fixing glaring holes in the base game for years to come instead of building up on a solid base.