Darrow Kirkpatrick has a taxonomy of retirement calculators here: https://www.caniretireyet.com/the-best-retirement-calculator...

These calculators would be in the low fidelity category. He gives examples of others that have higher fidelity.

Bogleheads has a nice list of calculators also: https://www.bogleheads.org/wiki/Tools_and_calculators The Retiree Portfolio Model is a high fidelity spreadsheet tool that takes some effort to use, but can be well worth it: https://www.bogleheads.org/forum/viewtopic.php?t=97352

One thing that low and medium fidelity calculators could do to provide value is education. These calculators are mostly used by those that are years away from retirement, who are probably younger and could use pointers to topics to learn about that will apply to their situation going forward.

For example, a colleague asked me about backdoor Roth contributions recently as he now makes too much to contribute directly. It turns out he had rolled a 401k into an IRA when the company we worked for was sold 12 years ago. I left it in a 401k and told folks at the time about the rules that might make backdoor Roth contributions uneconomical. I failed to educate properly at that time, and now he's not able to do Roth contributions to save money in the future.

Calculators that grow with you, remembering what you've entered and prompting for only the information needed for the next decision point are also a good idea. If you are targeting engineers, you'll likely grow into a very high fidelity calculator towards retirement, as engineers tend to like to fully understand the nuances of the situation. It may be that your tool creates a spreadsheet that can be downloaded and edited to handle the last 5-10% of the special nuances for that particular individual.

Making data input easier for users to think about is also a good idea. The recently featured on Hacker news https://news.ycombinator.com/item?id=23069276 calculator has a place where you can enter special future spending. Having a page that you enter your child's age or rough birthday (even future) could make prompting for uneven income and spending like not working, child care, sports and college easier for folks to plan for and enter.

There are many cliffs in the tax code for example. Frugal professor had an old post that shows some of the cliffs before the 2018 tax code changes:

https://frugalprofessor.com/federal-income-tax-calculator-to...

I'd like to see calculators that determine which cliffs folks are likely to be near and educate them about the issues.

The ACA health care subsidy cliff is another important one. Earn $1 too much and your health care costs go from <$100 to $1000+ per month for early retirees.

Getting a balance between tax deferred (traditional IRA, 401k, etc.), tax free (Roth) and taxable savings is also important. You want to be able to withdraw savings from each of these in retirement in a balanced way. https://www.gocurrycracker.com/6-years-of-nearly-income-tax-... has a nice rundown of what they do to minimize taxes.

Why am I talking about taxes so much? The only thing you control in your retirement is the contributions and withdrawals from the various accounts. The market conditions control the returns. Being smart about how you balance your actions to minimize tax drag can be a big boost towards retirement.

Other decisions can also have a big effect your retirement spending. Are you planning on retiring in a state that taxes your social security benefits and capital gains? How much sooner could you retire moving to a state that doesn't tax these?