This has been discussed a few times. Theresa, is an Archon. So was the hero from the first one. However not all decendants gain the immortality power, only a few do. So the hero died and instead had a family and your his desendant. Which is why Theresa helps you here and there your the equivilant of her great great great great great great great great, niece/nephew. So the only family left. Her master plan is to take control of the Spire. This is to undo all the damage done to her family from Fable one. She however needed you to kill Lucien and stabalise the situation to gain control. As she's immortal she has all the time in the world to wait. She allows you to have your wish which depletes all the magical energy from the Spire. She however gains the Spire and is currently waiting for it to recharge. Like a rechargable battery the Spire expends all it's magical energy in one wish for each person, she's simply waiting for it to recharge, so she can use her wish. Which from various hints and sources in game seems to be wishing her brother back into the world, along with all the original charaters etc. I think this due to the book on doom mongerings "the end is almost nigh" if you read it it gives details on the dragon fight and Spire possibility. all yu have to do is use your imagination and a few other hints the game gives you to make the leap to Theresa.