Penis statues. Everywhere. Like this-

Make it happen, Bioware.
this, and also add the option to remodel any towers into giant penises... and any and all silverware should also be in the shape of penises...also the hilts and handles of weapons... instead of tower shields, penis shields...penisssss~ 8===D
This is not decoration per say but what I think should be in Skyhold.
- Moon Door.
- Erotic Statues and Tapestries.
- Library with all types of books including friend fiction.
- Secret Passageways.
- Aviary to store birds for communication
- Eluvian for fast travel.
- Danger Room to practice combat. A medieval version of the X-Men one.
- Experiment Room to test new explosives, potions and poisons.
- Well stocked bar with poles for pole dancing and stripping.
That's just about it, I think. 
1. A nice idea, doubt it'll ever be added
2. Doable and probably will be an option..probably. (Exotic will be more likely rather than Erotic)
3. Library is a given for any base in these types of games, not sure about the "friend fiction" though
4. Not exactly a decoration, not really needed I think.
5. hell yes, owls or ravens please~ I'd say pigeons but with Shale roaming around Thedas, our communications may or may not reach their destinations
6. Meh, they're rare and I doubt Morrigan would recreate whatever she did to make them work.
7. Why a room? The usual Training grounds would work fine.
8. A lab would be nice, wouldn't test explosives in there though.
9. ....come on now. 