I think that this idea is superb.
There is a huge opportunity for products aimed at families, where someone roughly of age 13-19, feels drawn towards STEM, and wishes to voluntarily "play" with STEM concepts in the summer---a time when schools (at least in the USA) are closed. This is analogous to someone of the same age who feels attracted to soccer or swimming, and who voluntarily devotes a few weeks in the summer to go to a camp dedicated around that sport. The young person would instead go online, and engage with Sage, math, physics, Python, chemistry, and perhaps some economics too (e.g. linear programming).
Many schools for that age group have a "math and science club" or a "STEM club" that could point young people toward these activities. These activities could also be enjoyed by young people on weekends during the school year.
The computer gives a lot to such an enterprise, as compared to a textbook without the use of a computer. There can be graphics, animations, and even encouraging sounds when a "challenge" is successfully completed. We can have videos from university (or school) faculty who have authored any particular lesson, introducing the concepts. Due to the pandemic, vastly more teachers and professors know how to make videos now, compared to 2018.
Of course, I won't be able to devote any time towards this while the 2nd edition is still in progress. The 2nd edition "owns" my time this summer. I can contribute time starting in September of 2021.
I think the key to success would be to have a medium-sized group of authors, a diverse mix of high-school teachers and university professors, making mini-projects and challenges in many subjects, perhaps with some peer-review (but not too much). The teachers and professors who volunteer their time should not be micromanaged on things like fonts and indentation. That sort of nit-picking about typesetting is a wet blanket that smoothers and asphyxiates creativity.
p.s. In case someone wants something right away, for an eager teenager bubbling with early-June energy and enthusiasm, I recommend:
- B. Averbach and O. Chein, Problem Solving Through Recreational Mathematics, published by Dover Publications in 1999. [Originally published by W. H. Freeman & Co. in 1980.] (400 pg.)
- J. Beissinger and V. Pless, The Cryptoclub: Using Mathematics to Make and Break Secret Codes, published by CRC Press in 2006. (215 pg.)
- G. Ellison. Hard Math for Middle School, self-published in 2010, but really good. (238 pg.)
- S. Gordon, F. Gordon, A. Tucker, and M. Seigel, Functioning in the Real World---a Pre-Calculus Experience, published by Pearson, 2nd edition in 2003. (800 pg.) Note: this is more suited to the classroom than to recreation.
- B. Kastner, Space Mathematics---Math Problems Based on Space Science, published by Dover Publications in 2012. [Reprint of Space Mathematics: A Resource for Secondary School Teachers, published by NASA in 1985.] (192 pg.)
- ... and since employers and universities alike are decrying the lack of "soft skills" (organizational and communications skills) in today's "under 25" generation, I would be doing a teenager a disservice if I did not also recommend: D. Allen, M. Williams, and M. Wallace, Getting Things Done for Teens---Take Control of Your Life in a Distracting World, published by Penguin Books in 2018. (288 pp, many of which are cartoons.)