Not only would it help let you know what you're about to mine, it would also let you realize when you accidentally have your tool set to 9x9 . I know there is AoE base safe mode but also sometimes it can suck to destroy cosmetic sections (with no tile entities) so overall this would be a helpful feature.

As in the title, the highlight doesn't have to be very bright. Something like the highlight from Builder's Wand will be enough.

Thats actually not a bad idea! I will add it to the todo list. (Not sure exactly when i will be able to work on it.)