Keeper of Night, I'm assuming you're talking about female Morgan. (The avatar's child is always named Morgan, regardless of gender, and is always the opposite.)
Your best bet is to marry Lucina, Aversa, Panne (debatable), or a manakete. The reason for this is that, since Morgan can reclass into anything anyway, you want to pass her something that no one else can get (Lucina has Aether, Aversa has Shadowgift, and manaketes have...well, the manakete class - though I think that means Morgan will be weak to wyvern-busting weapons in any class. Not positive on that). In addition have Morgan's dad (i.e. you) give her a skill only possible on a male class - I recommend either Wrath or Counter. (Axefaire is another one, though Counter is probably the best.)