I've been thinking a little about d6 alternatives to the standard d20 attack roll. Not that I plan on ditching the d20 in most cases, but I thought I'd consider the options.
The most obvious would be to try to keep as close to the d20 range and roll high methodology as possible. That would be a 3d6 roll (3 to 18 range.) What would you call this? Target 3:18?
The behavior of the dice would not be the same, though. You would have to live with the fact that the bell curve makes armor differences more pronounced, and not all probabilities would be close to the d20-based approaches. AC 9 would have higher odds of success, AC 7 is about where AC 9 is under the standard approach, AC 2 is very hard to hit.
Adding a die to make 4d6 would still involve a bell curve, but you could keep the target numbers in the official table or use Target 20 and not mangle things too much. AC 9 would still be more likely to succeed. AC 2 would not be as difficult, though. The downside? You would call it Target 4:20.