Since there is a problem with story progression, (Sims don't marry and have babies autonomously), would it be ok, BEFORE you actually enter your game, and while you are in Edit Town Mode, to go into the various households and have each household with couples try for baby, leaving the female pregnant? You then finish up placing whatever venues, change from household to household to basically impregnate the entire town, choose to be out of all the households, save the game and then make your character and actually enter play?
To be clear the steps you take would be:
1. go into your town and choose Edit Town.
2. Place any venues or items like bookshelves.
3. Select any household and try for baby.
4. Go into Edit Town and choose a new household. Try for baby. Rinse and repeat till all females are preggers.
5. Remove yourself from all households and save game so your town is now edited with all females pregnant.
6. Create your Sim and begin to play your Immortal Dynasty per usual.
I'm thinking this would give a bigger gene pool without having to have someone in your own household going around "pollinating". Of course, it would only work for the first heir but it could be interesting.