Powershell ISE for Linux?

Not available yet, and may not ever happen. According to David Wilson who is responsible for the PowerShell IDE tools (ISE and VSCode PowerShell extension), the ISE relies on some components which are not yet open source and so can't be used with cross-platform PowerShell. You can see his response in the comments section on this blog post:

https://blogs.msdn.microsoft.com/powershell/2016/08/18/powershell-on-linux-and-open-source-2/

He states that they hope to release it eventually, but realistically it seems like all the development effort of late has gone into VSCode and that this will be the go-to editor for PowerShell on all platforms in the future. Time will tell, I guess.