You know, one thing I'd like to see is the ability to marry non-nobles, especially party companions (who you're likely to develop more of a bond with compared to a wife/husband where your bond comes after marriage rather than before). This would definitely alleviate the issue of not being able to arrange marriages for daughters/women without it being (mostly) a loss on your end.
I'd say the main issue with bringing husbands into your clan is that NPCs would need to do the same thing or else it'd be a one-sided advantage for the player character.
My proposed solution would be, if a clan has only daughters, that one of the daughters (such as the eldest) be treated as if male for the purposes of arranging marriages (which is to say, since they're the likely clan heir, they bring in the husband rather than have to wait to become clan head before doing the same thing). Otherwise, the Hermit's Retreat is something I'd like to see revisited since it'd be nice if you could retire your player character but keep them around as an NPC in your clan. It'd allow for more control over succession without going so far as to completely retire your character, who could at least be a governor or a steward/doctor/scout/engineer for their heir player character.
Perhaps a mechanic where an heir is designated prior to the death of the clan head could enable the above so that, should you designate a woman as your successor, she can get married and bring in her new husband rather than leave to enter his clan. In terms of real world historical precedence, I've seen this several times in Japanese history whenever there's a shortage of eligible male successors, most famously with the Tachibana Clan in the 16th century. It's also a great example of "semi-retirement" since there's several cases, such as Hisamasa Azai of the Azai Clan, retiring but still effectively the leader, or co-leader/chief advisor, as the guardian of his teenage son Nagamasa (who was up-jumped due to martial popularity in contrast with Hisamasa's own unpopularity among the clan elders).
I agree at birth of the children you should be able to assign an heir, natuarlly when you have 1 kid, it would be that one, but when there is more you can pick who you want(some cultures had such things in real life, for example in royalty, where daugther is only the heir until a son is born, then he is the crownprince).(Example here in Norway with the royal family in this current king's heir. Mærtha was in line until Håkon arrived some year later, the Swedes on the other hand changed their hereditary laws, so they have Victoria(1st born) to be the ruler, and Carl Phillip(born later) missed out on sucession as such(had they not changed the laws, he would have been the heir).
As for marrying non-nobles - this is taken care of on pc with mods though.
There is several mods that lets you romance and marry those non-nobles.
Tbh unless they "change things" TW "lied/decived" per se if you want to stretch it, as in the early footages of the game, one of the things they "showcased" was actually meeting a random wanderer and haveing a romance and wed them later on + kids etc.
(I'm sure it was no deception by TW, but they oversold it to us the players, I among others was abit cranky that its not in the game).
As for "retire your starting character without the Hermits retareat" this is actually done by mods(again..)
The mod lets you choose whichever clan memeber you want to control.
This could be spouse, kids or even "wanderers you have hired".
Retireing your main, to have them be a steward is something I'd love to do, or an option to take control of characters, as some of them is "useless" in combat kind of, so its abit tedious to level up their combatskills, vs if you where in control and dominated with them.
Leveling combatskills is much easier now no doubt, but still some of them is abit "harder at start" as the penalty for ranged etc is very high imo at below 25(read the wanderers skill should all be 25 in all skills as minimum, so they arent useless in some things).(most of the other skills is easy to lvl them up in ultra-fast. (steward, scouting, is superfast)