I have a few ideas on how to make legacies and any other tvd related project succeed if they return, some of it Ive said a dozen times, some might not be
In no order of importance-
First off as stated many times there needs to be a danger to the protagonists that somewhat lacked on legacies, I think we need to move beyond how many protagonists that can be killed off, I think its been established as bad as that is, to make a true villain they should do a worse fate than death, I dont think that would happen however.
Stay away from the complicated mythology, luckily it wasnt too bad with the gods stuff although a few times I think I was like huh, I mean even the fury stuff was confusing. But even a fantasy nerd as myself cannot stand this overcomplicated mythology. I almost wanna say throw away all the rules at this point cause there isnt 100% consistency. Im not saying you have to make this franchise flat out stupid but lets remember this is still entertaining art first not homework.
Do not bring back monster of the week, it was a failed experiment, I guess they could still incorporate monsters but they were never handled greatly, I mean idk, I think maybe they should just bring in shapeshifters as the only new species left and call it a day.
Answer any remaining questions that are too vague and flat out need visual explanation. Some can be answered via clues but some you have no choice but to show. And tbh Im not a fan of fans(no pun intended) having to fill in the blanks for the writers, it kills the reason we go to see tv shows so someone else can do the great writing for us and we tune into see it.
As usual try to dial back on plot errors or just even common sense errors, like I'm a bit more lenient on errors in the magic stuff cause its hard to put realistic rules on a subject like magic, its hard to get mad at inconsistency. But some stuff like cleo having the power to inspire ideas in people is not good of an idea due to common sense, stuff like that.
Sort of going back to the answer questions bit, they need to take full advantage to dive deeper into villains motivations and personas. They simply didnt do a good job at explaining the motivations of malivore, ken and necromancer as to past villains, there was still vagueness within them. That bothered me alot.
No more filler, that is an extension of no more monster of week but like no more therapy box/hallucination episodes
I still think the human characters at best need to be guest stars simply for the fact this is a show/franchise about the supernatural not the natural. Its also established in tvdverse the supers are in the minority compared to humans.
I am not gonna say we need more cameos cause unless its to do with the story then I dont think they are needed.
Imo these are the main issues to cover in order for legacies and potential spin offs to succeed so they dont mess up again.