Why not send a Gaia-like mission to Mars? This answer to the question why we didn't send Gaia to Neptune's orbit raises the question of why we don't send a Gaia-like mission to the orbit of Mars. It seems like it doesn't have the problems mentioned in the answers, and it would make it possible to measure parallax for stars 50% further. 


orbital speed: waiting 6 extra months to get better precision seems like a better trade off than waiting 9 years to get the same precision as Gaia;
partial coverage: after two years, the entire sky could be covered. Far from the 168 years for Neptune;
telemetry: sending back data from a Gaia-like mission can't be that much harder than sending back data from any other of the tens of missions to Mars;
power: the solar panels don't need to be 900 times bigger;
radiative environment: contrary to Jupiter, Mars doesn't have a magnetosphere, so the spacecraft wouldn't be disturbed by it.
The panels need to be about 2.3 times bigger to generate the same power.

The telemetry requirements are limited by distance to some power, assuming that the communications beam cannot be infinitely narrow. Mars is much further away than the L2 point - somewhere between a factor of 50 and 250.

It wouldn't necessarily make it possible to measure parallaxes 50% further. Gaia works by taking approximately 70 (on average) measurements of a stellar position over 5-6 years. This amounts to sampling the 2 au baseline about 10 times, thus reduing the error bar on a single baseline parallax by about $\sqrt{10}$ and making the "equivalent baseline" about 7 au.

To get similar precision with Mars you would need to be able to complete the 3 au baseline about 5 times, which would still take 5 years. So you don't obviously win in precision terms.

Overall I think it is the telemetry problem that is the real killer. Gaia is currently telemetry limited - that is why there is a limit on the number of stars it will measure and on the number of stars it will get spectroscopy for.