I'll put my theory under an fake cut so people who don't keep up can remain unaware. (j/k)
Since I'm such a royai fan, (please hold your anti-fandom bricks 'til the end) I like to collect and come up with completey random theories of what Wrath plans to do with Riza. (Please keep in mind that these are collected theories and most do not reflect the opinion of the writer)
A.) He's just holding her to make Roy obey him.
B.) He'll tell her that she can't have her guns with her in the office, for security purposes, then have someone attack her.
C.) He'll have someone 'have their way with her', which would really tick off Roy.
D.) He'll have someone 'have their way with her' and get her pregnant so that they have a baby to use as a sacrifice/key/other.
E.) He'll take her to Father to
1.) be turned into Lust/Gluttony(don't ask)/other Hom.
2.) be used as a human sacrifice
F.) He'll kill her in order to force Roy to perform a human transmutation.
G.) He'll threaten to kill her unless Roy opens the gate for him.
I'm going to go ahead and discourage the 'turn into Lust' theory, as most have already proved it wrong and tend to dislike it.
The rest is up for debate.
I'll stop for now. (If you're still holding your bricks feel free to throw them now.)
I'd like to hear your opinions on the subject. Any takers?
Edit: In regards to G., Wrath did tell Father that he'd get Roy to open the gate. http://www.readmanga.com/view.php?currtitle=1&chap=040&img=8