PCB ground connection for shared power/data bus system
Shall I connect the GND of the bus to my GND via balun or what?

Conducted emissions (Rf through cables) can be stopped with ferries or inductors. However placing a baulun or other impedance on the ground to block high frequencies can have unintended consequences.  Ground inductance can create common mode voltages (ground bounce) from fast switching currents from loads (especially digital loads) on a device. It might also be best consider the inductance to be placed on each device instead at the power supply as conducted emissions (RF) could travel from device to device.

RF circuit boards should always be laid out with a ground plane
connected to the negative power supply. If this is not done properly,
obscure circuit behavior might occur. From a digital designer point of
view the reason to this might be difficult to understand, as most
digital circuitry functions very well without a ground plane. At RF
frequencies even a short line will work as an inductor. As a coarse
rule of thumb, the inductance will be about 1 nH (nanohenry) per mm of
length. At 434 MHz a 10 mm PCB line will then present an inductive
impedance of 27 ohm. If a ground plane is not used, most ground lines
will be longer than this and the RF circuit board will almost
guaranteed not be functional.
Source: https://www.eeweb.com/app-notes/download/1655

I would include the option to include an impedance separator (ferrite or inductor) and test the antenna performance. If the antennas work better with the impedance separation, then use that.

My layout has a PCB antenna that is tuned for a specific ground area.
When I connect my device to the bus or when a new device is connected
to the bus, it'd increase the ground area, so that it'd effect the
tuning of my antenna, is it right? If it'd be the case how can I avoid
it to happen?
My system shouldn't be effected by noises from other system or
vice-versa (EMI related issues)

One problem you could have is high frequencies escaping through your RF section. It would be wise to separate the two with a star ground on the device. Make sure the ground plane for the RF is continuous. This may be a good way to route the power on each device by separating the digital and RF sections.

Source: https://www.eeweb.com/app-notes/download/1655