My opinion. Don't.
Upon the sage advice of a well-respected agent, I invested over a year in writing to the series market. He liked the protagonist I created. I liked her, too. And publishing houses like series.
Several well read authors follow the series blueprint successfully. Clive Cussler has Dirk Pitt. For James Patterson, it's his detective Alex Cross, and Janet Evanovich writes Stephanie Plum.
The second manuscript in my to-be series featuring the same protagonist has only one slight problem. It sits, unfinished.
I use the tool of character boards. I have a plot in mind, and maybe even a detailed story outline, but before I can proceed to write a single page I must first have intimate knowledge of the characters I'm writing about. I have to love them and hate them before I can share them with my readers, or they aren't going to love them and hate them. I need to know my characters so that my readers can see, hear and touch them as I do. It has taken you a long time to find the forever-friendships in your life. You have spent years growing these friendships into a deeper existence of 'Namasté' . I need to speed things up a bit.
The process goes like this. First, I paste a barrage of photograph clippings from old magazines that closely resemble the physical attributes of each character. Then I interview each character, even secondary ones. I need to know where they went to school, where they've lived, what they graduated in, what their hobbies are, and what they like for dinner. I drill down further. I learn their quirks, their regrets, their nightmares and dreams. I need to know despicable things about the sweetest girl in class. Only then have I dug deep enough to discover the authenticity of GMC [goal, motivation, and conflict].
It doesn't take long to discover your enemies, does it? But to truly understand their GMC, the writer must treat them with the same amount of authentic intimacy. Sometimes that means finding a thread of tenderness in evil.
The parting of ways with a beloved or despised character is always bittersweet. But, as when I moved from my home state of Colorado, I took my beloved friends with me in my heart and soul, and all of what is me. Namasté .
'No tears in the writer, no tears in the reader. No surprise in the writer, no surprise in the reader.'
For me, I lose an element of surprise and emotion when I continue with the same characters facing new sagas.
Then again, there's my second humble opinion. Never say never.