The goal is to make this into a two story coop. We'll put walls around the deck to make it the ground floor of the coop. Underneath, we'll fence it up with hardware cloth, and that will be part of the run.
It is bigger than it looks. Just eyeballing (and I'm really bad at eyeballing measurements so...) maybe 8' by 8'? And the ceiling is tall enough that I can stand up straight in there. Maybe 8' at the roof's highest point. And that's just the main "house" part. Walling off the deck will make it about twice as big.
The hatch could make a great access door for the chooks to get to the run below during the day. Lots of reinforcements to make though. This playhouse is a favorite of squirrels, chipmunks, mice, and groundhogs. As you can probably tell by all the nut shells laying around

and lots of prey means lots of predators sniffing around, too.
Below on this side, we could make it one big door for cleaning.
The tree growing through the deck needs to come down if we're going to go through with this. And there's a lot of brush removal and cleaning up to do around the area. So many fallen branches and trees (which is something else to think about— definitely don't want huge branches or trees coming down on the coop and run).
There are several good spots for nest boxes, roosts, and ventilation. I'm thinking we'll keep the front window, close off the door where a slide used to be attached, and nest boxes will go on one of the sides. Keep one or both of the side windows for summer and make it so we can shut them in winter. More ventilation higher up.
More pics from inside. The back (last pic) we could leave open. It'll look, feel, and be more roomy. I figure the chooks could use that railing as a roost, maybe (not the only roost, obviously).
You can't see the deck because it's under a layer of snow and ice (which I tried to clear off with my boots to no avail

) but if you follow the rope you can kind of see the other post which rotted through and broke years ago, and that's the other edge of the deck.
The swingset portion I think would be perfect to build a run off of, which they'd access from under the playhouse. Plus with another hooman access door, maybe at the A-frame part?