As of now, animals really don't like to be penned in. Instead of being happy, content, dumb animals they're dumb, dumb, dumb animals.
They repeatedly headbutt any non-solid block keeping them penned in, whether it be gate or door. If they're in a 100% solid enclosure they'll move normally, sticking around the middle like you'd expect. But put in a door, they'll see it as nothing and try to walk past it.
This is obnoxious because when you try to enter your pen, whether to breed, collect eggs or slaughter some of your stock, they'll beeline towards the exit and spill out as soon as you open the door. And seeing as animals and wolves alike have a tough time getting through those almost-a-block openings created by gates and doors, it's a real pain in the blocky back to force them through with wheat.
The only new block or item I'll suggest is a "Double Gate", 2 blocks high and wide. It creates a nice, wide opening that integrates with fences so animals can walk in without getting stuck. It opens like those tool-booth gates, with the arm that swings upwards so that the arm doesn't extend a block out.
tl;dr: Fix up animal pathfinding so that they don't rush a closed gate/door, make it so they and wolves have an easier time getting through single doors and add double gates for easier corralling.
While I think a two-wide fence would be pretty neat, you can kind of make it yourself using two doors (iron or wood) and the tiniest bit of Redstone knowledge. Basically, I think all you would have to do is have a lever and a line of redstone dust that goes under the blocks that the doors are on top of. The lever will toggle the doors between open and closed.