I have had ideas for a new crossover and from scratch with different premises; can I please ask what you think of them and I will say that before anyone asks: I won't be able to have Lucy in a main or big role. Sorry as I know she would be warranted and she definitely would have strong reactions but she is a character that I don't think that I can write in a main role. Same with most of the Loud sisters personally. Plus no Lincoln X Hellaverse pairings as I may do him with a LL character or an OC Hellborn. Here are my ideas:

Lucy does end up finding an actual arcane book and puts down a ritual to summon the king of the demon realm with Lincoln helping; Charlie ends up being sent instead (Lucifer was busy and she volunteered to fill in as she always wanted to meet humans) but thanks to a mishap with the ritual: Lincoln ends up being bound to Hell or ends up infused with his magic and so Charlie has to take him back to Hell so that they could get it under control.

Turns out that Lincoln was adopted and he was born a Hellhound and Loona had sent her young newly born brother away to Earth; finding a human disguise after their birth parents died so he could at least have a better life. She was caught right after sending him away.