Hi everyone, new member here, just joined :) I’ve been following this series with great interest from the outside and wanted to contribute!
I think I would vote for Osgiliath as this line up currently stands because of the opportunity for more important unique characters (Faramir etc) as has been said by many others so far...
However, a thought has occurred to me after thinking about the LEGO Movie line set 70813 that a Mûmakil could be a part of a broader “Mordor Cavalry” set, which could have a brick built Mûmakil but also include all the contents of the Witch-king vs Eowyn set we all want. That way we can get a molded fell beast in a set with enough ‘bricks’ alongside to justify the large molded creature and minifigure count etc.
I'll keep my vote for Osgiliath at the moment, sorry if I distracted too much...
(As a side note this is my LDD MOC of a Mûmakil design: http://www.mocpages.com/moc.php/367145 )
Happy to be part of this community!