Well, any series, mini or otherwise, needs some planning.
Or so they tell me.
Howl was not intended as the beginning of a series. Watch Me is the sequel, but my publisher insisted it needed a series title. "Pack Justice" was was born.
That's not how it should happen.
There's a reason why it shouldn't happen this way. Several, actually.
The #1 requirement for a series is consistency.
If you don't have consistency, your readers will flounder. Worse, you will flounder.
It's not a case of just making it up as you go along. While it works for some people, most writers require far more structure.
Let's face it, we're not computer brains or have the memory of an elephant. We're human, and we forget stuff.
That's not a terrible thing–unless you write by the seat of your pants and what was established in book 1, is forgotten in book 2.
If your vampires sparkle in book 1, then they should still sparkle in book 2.
Basically, be very sure of the rules, locations and circumstances your characters have to deal with. It's a world you create, but that doesn't mean the historical family seat can suddenly move to another place in another book. If you established "Kirkwall Castle" as a 15th Century fortress with 12 towers and a fully functional moat, located in Scotland, in book 1 — then that's where it stays. It can't suddenly turn into Kirkwell Mansion and only have 5 towers, or suddenly move to Wales, in book 4.
Now that seems obvious, but I assure you…I've seen it.
The little things matter. A certain turn of phrase by a main character who shows up in other books. The name of the nanny three brothers have grown up with. It won't do if one calls her Mary, the next calls her Anne and the third claims her name was Beth.
Yes, it takes a lot of work, and you better sit down and make a ton of notes–and organize them for easy reference, because you will be referring to them. Often.
You need to world build right from the start and have a good idea of what is where, who is who, and who can do / does what.
Readers notice, and if your world becomes inconsistent, you will lose them.
I have a series I've been working on for a long, long time. I've world built for years, pared it down, ramped it up, ditched and reinvented. I know where everything is, I know the characters backward and I'm completely aware of where they fit into the scheme of things. I've built timelines, researched abilities…you name it.
None of those books are available yet, because I'm still making sure things are consistent and fit in with each other.
So if you're thinking of writing a mini series — or any series at all — then make sure you do your groundwork first.
Everything else will fall into place if the foundation is solid.