Alistair becomes king

The most obvious way to seize power of Ferelden is to have Alistair claim his birthright as king of Ferelden, much like Aragorn does in Lord of the Rings.  This can be done one of two ways.

Alistair as king of Ferelden

One can have Alistair marry Queen Anora, creating two birthrighted rulers ready to seize power from Loghain.  However, depending on what you have done throughout the game, Alistair might not be willing to do this.  For example, if you are playing a Female Human Noble and are involved in a romance with Alistair, he may be more inclined to marry the player and have her serve as queen instead of Anora, which is completely possible.