Sorry, but this is silly, over-complicated and counter-productive.
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Complicated instead of simple. If dare to add every single theme that exists you would end with a ridiculously list of over 500 themes, most which would give 1-2 results. Break "comedy" (for example) into 10 different themes will only frustrate people and make harder for them to find more content.
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Now search for manga is more difficult. Since the last reply is about
Umareru seibetsu wo machigaeta i'll use it as an example. Before all you had to do was search "gender bender" and you could find it. Now you just have to browse because it's not tagged as neither genderswap nor crossdressing because it fits neither.
Congratulations! Now you made it harder to search for. "We will make a special tag for it, all problems are now solved!!!!" works...until you realize that, like it, there are plenty of not-mainstream works without a relevant tag. It's almost as if gender-bender was an umbrella term to search all manga with that description! There are other more like this one too...
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You end with deceiving results. Let's see the themes. "Cooking", what a nice theme huh? Let's see which works appear.
Hakumei to mikochi? There is very little cooking on this one, many "arcs" are not even related to cooking.
Silver spoon? That agricultural manga with close to zero cooking is now about cooking?
Dragon ball?! LOL. "But they had cooking in 1 chapter so it counts!" seems to be the criteria used here. Using that logic, almost all manga should be given the "fashion" theme since at one point a character wears fashionable clothes. The idea alone is silly.
This is a classic example of something that sounds good and like an improvement in theory but awful in the practice.
Now the search tool is harder to use.
Sadly, i don't expect people in charge to listen.