recommend me an open source Nagios alternative

[HarengaWoltraz]

We are using Nagios because it's there since years and I'm looking for better alternative for it.

My requirements:

Email reporting

Basic User Managment (Ldap / AD would be a bonus)

simpler configuration than nagios

Client with not that big hardware requirements

Bonus: A system where the client pushes Notification to the monitoring server

My environment:

Servers: Bare Metal, VM + Docker Images, I'm preparing Kubernetes

OS: Linux + Windows Server,

Network Local Network + Internet.

Distribution /Configuration: Nagios + Nrpe Configs with Ansible

Monitoring Tasks: System, Network, Http Checks, basic Application Checks

What would you recommend me and why?