I was just talking about this the other day with my group. We play D&D 3.5 modified with a few rules from 5.0. We came to the conclusion that any luck attribute would have to be an overall bonus to all rolls and as such would be OP as an attribute. Everyone would make that their highest attribute nullifying the need for such, while adding a need for the DM to raise DCs. It would also lower the importance of the other six attributes and trained skills. In the event that you need a straight luck roll, just use an unadjusted d20. If you want to make it more interesting, add the inspiration dice system from D&D 5.0.