Maybe I'm just nostalgic but I think a classic IPA doesn't need a modern twist. I'm all for IPA open sourcing their beer; heck, free beer is good enough for me.
In all seriousness though, I already saw a user recommend kanidm. I can vouch for kanidm; written in Rust, it allows offline authentication and offline caching of user info, which is really handy if you're in a situation with poor internet connectivity. kanidm is feature rich:@g5pw already mentioned OAuth2 support, LDAP, RADIUS; etc. It even supports TOTP!! Kanidm doesn't support SAML IIRC, But SSO can be achieved through OAuth2 with OIDC.
From kanidm's Github:
Kanidm aims to have the features richness of FreeIPA, but without the resource and administration overheads. If you want a complete IDM package, but in a lighter footprint and easier to manage, then Kanidm is probably for you. In testing with 3000 users + 1500 groups, Kanidm is 3 times faster for search operations and 5 times faster for modification and addition of entries (your results may differ however, but generally Kanidm is much faster than FreeIPA).