I have two buildings per each of my factions. These buildings offer upgrades and 'extensions' which are buildings that are produced by the building as 'building pets'. In older versions of GAE I would trick the engine to building them in specific areas using cellmaps, though in more recent versions this trick stopped working.
Here is an older screenshot showing the spectral gate (near the corner of the walls, the portal like building with two large pillars placed in front of it).
[img]http://img59.imageshack.us/img59/9277/mrisenecribus08.png[/img]
Here is a newer screenshot of the Spectral Gate and the Hell Portal, instead of having their pillars placed either side of the portal, one is place to the right then one in the middle looking wrong (bottom left image).
[img]http://img829.imageshack.us/img829/2378/mrisehyperion10.png[/img]
I'd like to suggest produce skill/command options that allow you to set what direct from the unit that other units are produced from, this should also be relative to the unit's rotations for buildings that rotate or units that summon pets which will rotate a lot.
Something like:
<command>
<type value="produce"/>
<name value="pillar_of_the_dead" tip="pillar_of_the_dead_tip"/>
<image path="../pillar_of_the_dead/images/pillar_of_the_dead.bmp"/>
<unit-requirements />
<upgrade-requirements />
<produce-skill value="pillar_skill"/>
<produced-unit name="pillar_of_the_dead"/>
<produce-possition value="southeast"/>
</command>
Here <produce-possition> has a value that determines where the unit is produced from, I've put southeast but maybe specifying an angel would be better where 0 = east (though if the unit had rotated 90 degrees itself before producing a unit this would be south instead).
I hope this makes sense, it's nothing big, just a drop of perfection for my extending buildings!