Here are my thoughts.
FSX is a very capable scenery engine- Apart from a couple of issues related to the period in which it was developed (i.e. lack of proper multi-core threading, under utilization of GPU, 32 bit pipeline etc) it still holds its own in the current day IMO, not least of all because of the ability to upgrade most things within the simulator install.
X-Plane 'feels' more like a flight simulator than FSX does....but the scenery engine isn't quite up to par yet (the lighting is infinitely better than FSX and flying very low is generally more immersive but at mid-high altitudes, their procedural rendering can't match the realism that FSX's tiled aerial textures + Autogen give) and I really dislike the interface.
Flight had some really nice additions to the FSX code and would make the perfect base for a new simulator with the addition of global landclass, terrain and Navigation data- The lighting is far nicer (i.e. on autogen objects especially) and the flight model seems far better too....It would have been a hit if MS had simply provided their own marketplace where payware developers could have integrated their products into the 'store' and paid a small fee. This would have provided the post sim sales revenue that was lacking with the FSX release.....they swung too far the other way.
In other words- My ideal simulator would have the flight model of X-Plane, the scenery data of FSX, the scenery engine of Flight + procedural terrain refinement and lighting of Outerra.
Where does this leave Outerra? Currently Outerra has the capability to provide the scenery base- The new 'biomes' in development should allow more accurate representation of the world (i.e. landclass) along with the other parts in development (importing of vector data/roads, water bodies etc). As mentioned, the use of 3rd party physics/flight modelling libraries provide that portion of a sim and it can already import common 3D object formats. This leaves a decent proportion of the requirements for a flightsim not (yet) filled which begs the question, what should Outerra provide and what shouldn't it provide? If the consensus is that a fully fledged flightsim is too large a project for a small dev team, then the project moves to being split between the base engine (Outerra) and 'everthing else' (possibly created by the community)
Obviously 3rd parties can develop aircraft and improve the scenery/Airports...leaving items such as weather modelling, Airways/ATC needing to be developed within the 'Outerra Flightsim' (or at least the ability to add it)....this does run the risk, however, of a 'moving target' for development- I.e. the base engine breaks older data requiring it to be re-compiled.