Translation:Tomorrow we are going to think about our plans.
You can do research until you're pulling your hair out but the only way the use of prepositions will really sink in is as you use the language more. You just have to use repetition enough until it just sounds right. If you ask someone, why do you say pensar en they can really only say, that's just the way we say it, there is no real rule. The same for English, why do we say think about sometimes instead of think on, we just do. There are just so many different phrases that some of these prepositions are used in that you wouldn't be able to remember them all by just studying them, even if you could find a comprehensive list. Just completely remove from your mind things like ˝de=of˝, ˝en=in/on˝, ˝sobre=about˝, etc. Yes, those statements are true in certain contexts, but not all. Start learning phrases instead, ˝pensar en=think about/of˝. It's not that the word en here means about/of, it's just that when someone uses pensar en in Spanish it correlates to us saying think about in English. You will be less frustrated once you stop thinking how many different words por, en, de can mean and just start thinking about the phrases that contain them.
Sobre tends to be more physical; where things are in relation to each other in physical space. As someone said in a comment above, 'Vamos a pensar en nuestros planes' would be more like 'We are going to think on our plans,' which isn't a super common phrasing in modern English, but still totally correct.
Yes, it seems to be more idiomatic if you consider this link (https://idioms.thefreedictionary.com/think+on), but that may have changed over time and shows that the Spanish usage of "en" here makes complete sense.
The main takeaway is that languages evolve differently and some similarities may fade over time.
Better English to whom? Unless you think the answer is not proper English and is somehow wrong grammatically, your version is just another way to say the same thing.
"Better English" is very subjective and depends on a lot of factors like your education level, where you grew up, where you live, etc. It also spans a few countries that all have their own differences.
Best bet on Duo is to stick to the verbs that are given and as Daniel said considerar is the Spanish word for consider.
I feel like we all feel as if the Spanish language has limited vocabulary because a lot of us only know a small percentage of it. Spanish is just as expressive as English so if they wanted to say consider instead of think about they could have. We don't need to change what they say, they used the word they did purposely. Which is why yes, they "expect* you to use think about.
If a friend said this statement to you using think about would you correct them and say "I think you meant "consider', it's better English.
The following post is a bit long and (mostly) useless. You have been warned.
First, "Tomorrow we are going to think about our plans." is peak procrastination.
Second, it is absolutely normal to me to say 'think about' and I can live with 'think of' or even 'think over', although that may mean that we need to think again and maybe even change something. When I see numerous people complain that 'think on' is not accepted, I feel that I got in a digital equivalent of a prank/flashmob or that experiment where three 'participants' confidently say that one of the obviously equal-length lines is definitely longer, forcing a fourth (the real subject of the experiment) agree with them against his or her better judgement.