Honestly, I get that. I've merged a lot of my previous story ideas because I felt they were too similar and covered too many overlapping themes. Aside from probably being boring to read for prospective long-term fans, it's also boring to write. I can only complete so many comics in my lifetime, so I'd rather be efficient and not use 2 of my lifetime-comic-allocation to explore a certain topic when I can do so with 1.
That said, I don't think forcing diversity is a very good solution to this. Writing about stuff that your heart isn't into just to be different from your previous work would just lead you to having a diverse but shallow body of work, where each work is unlikely to stand out over other people's work of a similar nature because someone who actually wants to write those thing is more likely to do a good job at it than you are ^^;
Again, I recommend merging! If you have something else to say about a theme you're already writing about; don't abandon it for something different OR start a new story based on that theme (yet)! First ask yourself: "Can I fit this into a story I already have in the works?" Often you'll find the answer is 'yes', and it'll add more depth to your existing work and fill in some gaps that you were previously straining your brain to come up with solutions to. Amazing stories aren't built in a day; sometimes it takes years of accumulation to develop an amazing take on a theme that someone else would certainly have explored, but none as thoroughly as you have
This way you'll get a smaller body of higher quality work, which has the added benefit of being completable within your lifetime
Depth over breadth is the way to go, imo :]
Also this; a pattern in character creation or mood doesn't make the story more predictable because it isn't isn't a plot point any more than a setting is a story! For instance, my 3 stories that have a 'girl with a martyr complex' all carry that trait in fundamentally different ways that doesn't let me merge them without losing something important:
Anwen is your textbook Well-Intentioned Extremist who has dedicated her life to changing the world on a large scale
Dolly, in contrast, is a relatively powerless person with delusions about how far her influence reaches, and so isn't nearly threatening enough to have a similar role in the plot as Anwen
Cleria's whole deal is "I will protecc my friends' smiles", so she isn't going to do something world-shaking like Anwen (that'll get her friends wrapped in and make them worry about her), and she's too 'in control' and good at hiding her feelings to play a similar role to the more obviously erratic and unstable Dolly.
And don't get me started on 'adorable, quirky guy', which is so general that it can manifest in any number of ways XD