Roslindale Village Main Street had an open house at the hulking edifice in Roslindale Square, in part to solicit ideas on what to to with it. The BRA, which owns the long unused structure, has given the group and Historic Boston a year to come up with a plan. Even without giant transformers, it remains a fascinating - and dangerous - place to explore: One guide had to keep shooing me away from flimsy planks covering holes. It's two levels full of mysterious objects and spaces.