If by "tag suggestions" you are referring to code completion, then it is not the responsibility of NetBeans to provide code completion for Onsen UI tags. If it was done at all, it would be through an Onsen UI plugin that was provided to NetBeans.
If you navigate to Tools > Options > Editor > Code Completion and view the entries on the Languages drop list you will see an entry for Twig File. It exists because someone created a Twig Templates plugin for PHP developers, and it was approved by NetBeans.
Someone other than NetBeans would need to do something similar for Oren UI developers, but I can't find an Oren UI plugin for NetBeans, so unfortunately the functionality you are seeking simply doesn't exist.
If by "tag suggestions" you are referring to code completion, then it is not the responsibility of NetBeans to provide code completion for Onsen UI tags. If it was done at all, it would be through an Onsen UI plugin that was provided to NetBeans.
If you navigate to Tools > Options > Editor > Code Completion and view the entries on the Languages drop list you will see an entry for Twig File. It exists because someone created a Twig Templates plugin for PHP developers, and it was approved by NetBeans.
Someone other than NetBeans would need to do something similar for Oren UI developers, but I can't find an Oren UI plugin for NetBeans, so unfortunately the functionality you are seeking simply doesn't exist.