Sorry for the delayed reply but I was out of town and just got back today!
My recommendation would be to use NRaas Traveler, then you can travel on vacation to any worlds you have installed, not just to the three WA worlds.
But if you really want some interesting sims without having to travel, to limit file bloat, you could always check out sims wiki for each town and pick out the ones you like.
Then start a game in that town for the sole purpose of saving some of your favorites to the bin and then using them in your Immortal Dynasty game for each gen.
If that's allowed.
Midnight Hollow, Dragon Valley, and the greenies of Hidden Springs, can add interesting hair, skin, and eye colors to your family tree.
And some of the NPC sims in each town can have colorful genetics too, especially in Dragon Valley.
As far as keeping your saves stable, I would definitely recommend NRaas Overwatch and Error Trap, and cleaning out your cache files on a regular basis.
I've never attempted an Immortal Dynasty, but I am on town #13 in my Town Jump game, and I know I would have never made it this far without NRaas mods!
Someone on the NRaas site might be able to give you even more concrete advice on keeping file bloat down and your game in a playable state even with traveling.