It has been nice working with new contributors that have specific knowledge of such issues. The core team is two people, and we're lucky to have new contributors come with specific knowledge in different areas. So, ideally, we could wait for someone with domain expertise to come along. Failing that when have time we pick off the next most important-seeming thing and add it.
If this is only dozens or low hundreds of person-hours, then just do things at your own pace. Don't worry about releasing a viable product. I'd guess that you find adding tax forms fun. But it's probably not fun to update every form each year, respond to bug reports, and handle other support requests, all under the time crunch of tax season.
(As a side note, if you want real impact with a time commitment in this magnitude, consider being a VITA volunteer! https://www.irs.gov/individuals/irs-tax-volunteers)
Of course, if you have thousands or ten thousands of hours instead, you should be able to support everything I listed, plus the features you'd find in other tax software. With that amount of time, you should be able to do enough market research yourself to figure out what to prioritize.
* Forms 1099-R and SSA-1099 are unsupported. Retirees, or anyone that took a COVID-related distribution, can't use this.
* Form 1099-G Box 1 is unsupported. Many people took unemployment during the pandemic, and need to report that income on their return.
* Prior-year earned income for ACTC and EITC is unsupported. Many folks earned more in 2019 than in 2020, and could get larger tax credits by computing the credits using their 2019 income.
* Schedule C is unsupported. Many folks have at least some self-employment income. [1]
[1]https://quickbooks.intuit.com/self-employed/report/
As for software I can recommend to an end user:
https://www.irs.gov/filing/free-file-do-your-federal-taxes-f... https://www.myfreetaxes.com/
And for comparison, a more complete solution from an independent creator:
https://sites.google.com/view/incometaxspreadsheet/home