This is asking a bit much, I know, but I'd very much enjoy having an option to turn on relativistic effects in US2. I'm mainly thinking of those post-Newtonian approximations for general relativistic orbits, so that we could have objects orbiting close to black holes/neutron stars/etc. and still behave reasonably. I'll be the first to admit I don't even know the post-Newtonian equations, but it'd be a nice feature to consider.