You could do a sequel, but I'll suggest making the first book a sad ending.
If you do a sad ending and continue with a sequel, readers will like it more. They'll want to read the sequel because they're not satisfied with the sad ending.
In the sequel, you can unravel what's happened and even add a twist. Like the male lead dies in the first book, but in the sequel he could have a long lost twin that becomes the new male lead.
The sequel doesn't have to be so long, as long as you unravel what's happened and make a few scenes to turn the readers' emotions here and there.
You could do a sequel even after a happy ending, but that again depends on how good the plot was that your readers will want to read a second book.
You could do a series, with one book for each member in a group, kind of like what I did.
I wrote a Jirosé fanfic and the sequel is a Taennie fanfic. But no, I'm not continuing after that one.