So we have a converted attic. Now the master bedroom suite. In the attic the gas line sticks out an inch or two out one wall and is capped off.

What on earth was this for? The house had at one point a gravity furnace that was converted into a full blown forced air HVAC system.  

Was it a heater for the attic? Before forced air was added? No water lines so it's not a attic water heater.

More or less I'm gonna take it out, if it's for something I can't think of and I end up wanting one of the things I can't think of I'll leave it.

anyplace that there is, a joint connection or elbow, is a good place to drop a capped nipple if there is any chance you might need gas there in the future.  once the pipe is place it is much easier to just uncap and run line from that point. IMHO.

maybe for a potential attic apartment? my only guess. :)

How old is the house?  How big is the line?

Possibly for what used to be, or could have been an attic installed HVAC system? As stated, it's fairly common for anybody with foresight to throw in some capped lines in strategic places when running gas lines in a house. This makes it MUCH easier to go back and tap into a line after it's all in place and changes need to be made. If that is the case, they may have dropped a line into the attic just in case a gas-fired hvac system was desired in the future.

Old houses used to use gas for lighting. Sorta like Coleman lantern mantles in a light fixture.
They used space heaters too. Is there a spot on the wall for a flue pipe?

It's defiantly not gas lights. The capped off line is  8" from the floor and it's 3/4 pipe.

House built in 1940.

Space heater or stove...