Building an "Accountant-Verified" tool suite – What features would you add?

I’m a developer by trade and an accountant by background. I got tired of using 10 different websites just to get one financial task done, so I started building a single, fast hub for all niche calculations: Calculafast.

It’s focused on three pillars: Accuracy (verified logic), Speed (no bloat), and Utility (niche calculators only).

I’m launching tomorrow, but before I push the button, I’d love to get feedback from this community. If you were building a calculator hub for professionals, what’s the one feature you wouldn't leave out?