Well, one possibility is that they could bring insider knowledge. Lucius has been involved with the Death Eaters for a long time, so he might know their plans, hideouts, and secrets. Draco, having been on the cusp of becoming a full - fledged Death Eater, could also contribute valuable information. Their joining would likely cause a stir within the Order. Some members might be very suspicious of them, especially those who have lost loved ones to the Death Eaters. However, if they prove their loyalty, they could be a great asset in the fight against Voldemort. For example, Lucius's wealth and influence could be used to gather resources for the Order, and Draco's skills in magic, which he had been training for the Dark side, could be redirected towards the good of the wizarding world.
If Lucius and Draco join the Order in fanfiction, it would be a huge shift in the power dynamics. Lucius could use his connections in the wizarding high society to sway public opinion in favor of the Order. Draco, on the other hand, might struggle at first to fit in. He's been raised with certain beliefs about blood purity and the superiority of pure - blood wizards. But over time, he could learn to see the value in everyone regardless of their blood status. The Order would need to keep a close eye on them though. There's always the risk that they could be double agents, but if they are sincere, they could help in various ways like providing intelligence on Voldemort's movements and the activities of other Death Eaters. They could also help with undercover operations, using their familiarity with the Death Eater way of life to infiltrate their ranks from the inside for the Order.