[Update] Just a rumor, not substantiated, Forza is definitely not coming to PC at all.

With GDC beginning today, the spotlight is geared towards Microsoft, as they fully demonstrate the capabilities of DirectX 12 and it's implementation on Windows 10 and Xbox One. Reports from Worlds Factory, reveal that Microsoft might be preparing a PC version of it's Forza series. Microsoft, first revealed DirectX 12 at GDC 2014, with a demo Forza Motorsport 5, running on PC at 60 FPS. The session was titled DirectX: Evolving Microsoft's Graphics Platform. No further details were given at the time, although, Chris Tector, Software Architect of Turn 10 studios did mention that it only took four months using a four man team to get the game running. He also added:

We’re very excited to see console-style development coming to the PC through D3D12.

Tector will be present at GDC 2015, speaking during a session entitled Developing Games for Windows 10, which would make very little sense if they weren't actually doing Forza for PC. The session follows soon after Phil Spencer's The Future of Gaming Across the Microsoft Ecosystem. It could be possible that Phil Spencer is to announce Forza PC during his session, followed by Tector for more details on the title.

Microsoft dismissed Forza PC after last year’s tech demo stating to Eurogamer:

[It] was simply a rendering tech demo to showcase the power and ease of development for DX12. We have no plans to release Forza Motorsport 5, or any other Microsoft Studios Xbox One title on PC.

Fable Legends will be coming to PC and it's just the first of the major game franchises from Microsoft Studios

Things seem to have changed recently though with the announcement that Fable Legends, which is also a Microsoft studios title, will also be coming to the PC as well, thanks to DX12. The official blog Xbox Wire stated:

Fable Legends is just the first of the major game franchises from Microsoft Studios coming to Windows 10 and we will have more to share in the coming months.

https://www.youtube.com/watch?v=W1lHeLN1UGs

Forza Motorsport 6, is scheduled for release on Xbox One this holiday, so it would make sense for it to release on both platforms, although there is a possibility that they would be porting Forza Horizon 2 to the PC. We will know soon enough, and we will bring you any new information from Microsoft and DX12 as soon as it becomes available.