I'd go for I2C or SPI, several sensors and other devices are available with these interfaces. If you want a very reliable bus system, a couple of other bus or interface systems are available, like CAN, at elevated price.
One parameter is the device selection, which can be by protocol (I2C), select line (SPI), or by some more complex multiplexer (RS232/422). The master controller has to use the known address of each device, or a distinct select/enable line for each device, or it must switch (multiplex) all data lines for point-to-point interfaces (serial).
Another parameter is the signal level and interference immunity, affecting the allowed line length and transmission speed. E.g. a serial connection can use either TTL (5V) logic level signals, RS232 (24V level converters), or long distance RS422 (20mA senders/receivers), where the different sender/receiver electronics are exchangable without special coding. This exchangability applies to all interfaces using unidirectional lines (1 sender, one or more receivers), while interfaces with bidirectional lines (I2C..., each device can act as a sender at the same time on the same data line) require more sophisticated transmitters or level converters (3.3/5V).
I'd suggest that you start experimenting with the cheap and simple interfaces (I2C, SPI), to find out whether these match the intended environment (noise immunity, line length, transmission speed) and hot-pluggability (attach/detach sensors without system restart). Then you are free to build your own sensor modules; for other high-reliability interfaces you better use readily available sensor modules, unless you are very familiar with the construction of the related electronics.
Finally you have to select appropriate connectors, which can be specified by the bus system, or you choose your own connectors which may be universal or sensor specific (non-reversible). E.g. you can choose an specific (or coded) connector for each sensor type, with a feedback line (sensor connected, sensor type...), and provide a fixed number of dedicated connectors on your base station. Or you provide a number of general connectors, where any sensor can be plugged in at any place, or one or more connectors with the ability to line up any number of sensors (daisy chain). Outdoor applications may require rugged (water proof, shielded...) cables and connectors. You can offer multiple (in-/outdoor...) versions, if you want to sell your own system, or leave such details to the user.
But don't worry too much right now, start simple and improve your design later, until it fits your expectations 