We would be interested what requirements are there on the engine itself from such a perspective plugin/module, with regards to the thermals and turbulence modeling using terrain data.

I am not at all an expert in computer games or simulation, but here is what I know:

FlightGear has a fairly simple model which - as far as I understand - consists of horizontal wind which can be defined in strength and direction for different altitudes. It also simulates the planetary boundary layer, i.e. decreasing wind and a change in direction as you come near the ground. On top of that you can choose a turbulence factor which probably just adds a random component to the wind. Thermals are modelled as fixed vertical cylinders with rising or falling air. You can have a cumulus cloud on top of each thermal if you wish. All this is already quite OK to practice your flying skills.

Since Outerra is obviously a completely different league to FlightGear in terrain modelling, it might be interesting to look into more realistic wind modelling as well, like solving some form of fluid flow equations. From simple to complex, the buzzwords would be

1 Potential Flow: Static over time, no eddies, no turbulence, follows terrain, no thermals, very simple equation to be solved once (Laplace)

2 Stationary Navier-Stokes: Static over time, eddies, no turbulence, no thermals, quite complex but doable equation to be solved once

3 Instationary Navier-Stokes: Changes with time, eddies, turbulence, no thermals, complex equation to be solved every few seconds

4 Instationary Navier-Stokes with heat transfer: Everything, incl. real thermals - not quite the holy grail of engineering maths but almost...

#1 Gives wind that you can use to soar along a ridge, which is already great. But it will not model the wake behind mountains, which is a bore. Implementation should be easy, turbulence and thermals can be added as in FlightGear.

#2 Will also model the wake behind a mountain which is particularly interesting because these wakes make flying in the mountains tricky. Again, turbulence and thermals can be added artificially. My personal favourite in terms of complexity and what you could expect.

I am not sure whether current PCs would be able to cope with #3 next to all the other jobs in the simulation. And for #4...

There is an open source fluid dynamics library called

OpenFoam which you might want to check out.

By the way: You could combine #2 and #4 to give you a static flow field with thermals and only add turbulence - that would be quite cool. You would need to define ground temperature according to Sun illumination, ground colour, thermal capacity etc. to make something trigger the thermals.