@ThistleHope: No, there aren't any news on Icinga 2 for UCS 4.2.
@externa1: You have been asking about Zabbix. Have you sent a request towards Zabbix to support UCS? I'd be happy to help them going into the App Center.
I'd like to add some general words on how solutions are going to be available in the App Center. Univention provides the App Center platform and app providers provide their solutions. We are happy to accompany app providers on the road towards the App Center and we are interested in having good solutions available in there. Unfortunately, we don't have the ressources to take care of "every possible solution". I hope this is comprehensible.
That being said, besides letting us know about your need for certain apps, it is also good to request your need at the solution vendors, as well.