Would it really be so bad if we could use any character in any alliance IF at least that character completed the main quest line? I'd even go for opening it up if all quest, dungeons, bosses and Dolmens were completed throughout an alliance territory.
I mean, I travel the land and see NPC from ALL races within ALL Alliances. Some of them play major roles throughout the drama too. Could we at least open up for the possibility that a character from another area switched allegiances after spending time and spilling blood with them?