An air temperature gauge would be trivial to do, but it doesn't add much to the experience.I don't think Orbiter even has air temperature... Ah well...
Understand that the pros at FSX have mountains of money, teams of developers, and access to aircraft data that we don't have access to. They can develop their models to such an extent that they can be used as faithful system simulators.
Trying to get system temperatures that are realistic, meaningful, and match real world data is extremely hard to do from base principles. I did this professionally in thermal power plants for many years. Any subsystem temperature models in Orbiter are either going to be a quick and dirty approximation or an outright fabricated number generator.
I coded basic thermodynamic models of the engines that do a reasonable job of estimating the engine power. But without much better information, I really have no interest trying to spend mountains of effort making something practically guaranteed to be wrong, unless you want to discuss my consulting rates and have cash to burn.

