One that has tools that don't make your users hate you. Seriously, the open-source FPGA toolchains are breath of fresh air to use, despite being small projects with few contributors (although due to that and no vendor support they are severly limited in supported targets and special features).

Yep. The Icestorm toolchain for Lattice FPGAs is a real breath of fresh air -- fast compile times, multiple sets of interoperable tools, open file formats, development in the open... it's great. I just wish something like this was available for more than just Lattice parts.

FPGA development tools are generally dated, very very expensive, and one way streets for customisation.

From what I understand, open sourcing the bitstream format in its entirety will only do so much but it would certainly help. It's not just building GCC for FPGAs

Just better tools would be nice (and open-sourcing would bring some hope for that). FPGA tooling is atrocious, especially if you're used to software tooling. And the difference in tooling can sell chips all on its own.