Like I said, it's not the "We can change the map easily with some modding" issue regarding Valyria. It's the issue of IT BREAKING INTO MANY PIECES due to A FREAKING VOLCANIC ERUPTION that is the issue. Sure, someone already has made a restored pre-Doom Valyria for a submod for a version long ago. But the issue is a dynamic change from a non-broken peninsula to a peninsula broken up by the earth itself. I don't mean to sound like an ass, really, but a Doom/Freehold rising scenario just can't happen. For one, the obvious reason of a dynamic map change, but also because its government type. The Freehold wasn't just a merchant republic, it was a merchant republic co-ruled by forty wealthy, land owning families (Which the Targaryens were from). The game is based around one person ruling a territory. A multi-leader republic would be way too complicated to make for the level the Freehold was.
EDIT: Regarding the people for a possible Andal Invasion bookmark, we really only have a handful of names of the people who were important during the time, with the majority of who we know are really in the Vale. To make an Andal Invasion bookmark, we would need to break the continent down into very petty First Man kingdoms, and to do that, we would need to change the de jure territories, since the mega countries during the Seven Kingdoms bookmark really didn't exist during this time frame. We would also need to make families and members for those families (Made up and lore) to fill up the provinces, and finally, the Andal Invasion was a mass migration that took part as a result of the rise of the Freehold, which I just said if impossible to do. So, to make what you want would require a Westeros Only submod (That we have) to remove the Freehold obstacle, then have an event fire for well over 1 thousand years where you're constantly being invaded by Faith of the Seven fanatic doomstacks. And given that the lore's ideas can't be replicated in this game (Stranglehold of the Neck and all) PLUS the Andals being seafaring, the First Men would stand no chance. This is all combined with the fact there is no culture province shift and a VERY slow province religion shift. So what would probably happen is the invaders ruling over a people who rebel every month or just converting to the Old Gods and First Men cultures.