I decided to switch from current solution (using modified NUnit by our team some years ago NDistribUnit which run tests on VirtualMachines and then gather results on hub server) to Selenium Grid 2.
Option with ParallelizableAttribute was tried. Unfortunately I noticed that IWebDriver was stored in Global variable (puhh). This caused tests to start several browser instances, but tests used single IWebDriver -> tests execution happened in single browser -> tests were run under single process, but with several 'worker' threads. It was tried using 2 VMs as 'nodes' and local PC as Hub.
I know best solution is to change invalid idea to store driver in global variable, but it'll take too much time: there are 3k+ heavy UI tests to be updated; many static methods expected to have driver as global var to be updated also.
As well NUnit 3.0 provides Option to run several assemblies in parallel. To run several test projects it's good, but currently we have 1 assembly per one application. It would be nice to run tests for one application (one assembly) in parallel.
Is there other ways to use GRID + NUnit 3 here to make it work?
Finally, existed solution were refactored: now each test during execution has own driver. Due to this change a lot of code was re-written (it appears that too much methods expected to have IwebDriver as global variable)