Question: Hello, hypothetically speaking, if I planned a book series and it becomes a commercial success; is it a good idea to have a different main protagonist in each sequel? Or will it confuse the audience?Answer:
The problem is not that you will confuse your readers necessarily, but that you will disappoint them.
Generally, having the same main character helps tie a series together. There are many readers who, if they like the first book, will buy others in the series specifically to read more of that character's adventures. (This is why many series are named after their main character.)
Of course, it's not a hard and fast rule. Some series are tied together by a common story world (e.g. the Fred Saberhagen's Berserker series, or Star Wars
books). Some are held together by connections between characters (e.g. inter-generational series). You could even argue that when publishers put out a "line" of similar books by different authors that is a paler version of a series. In all cases, the point is to get more sales for later books by attracting people who liked the earlier ones.
However, the more in common books in the series have, the more likely readers are to buy the later books. Having the same main character is a big factor, and you risk losing some sales if you deviate.
On the other hand, if the first book is really well received, some people will buy the second just because the author is the same.
You may also find, if you get to this stage, that your agent or editor may advise you based on their knowledge of your genre.
How's that for a wishy-washy answer?