AHK1221 is working on a modification right now that allows the player to designate any number of vehicles to a group, followed by one or more locations, along with a spawn probability for each vehicle.
When you have multiple vehicles in 1 group all with a 100% Probability of spawning. What happens is that the number of vehicles that have the same probability is added, giving a total of the amount of vehicles there are with a chance of 100%. Let's say that 6 vehicles in a group have the same chance of spawning. How will the game decide which vehicle to spawn? Well, it will generate a random number from 1 to 6, and whatever number it lands on is the vehicle it will spawn, depending on where that vehicle is located at in the INI file. If it lands on the number 5, it will spawn the 5th vehicle. There is currently also a feature that supports more than one vehicle to spawn at a time. This feature is called Simultaneous Spawning.
The current features that need to be worked on so far are as follows:
Only one location can be added, even though the mod supports multiple locations, there isn't yet functionality to add in the second locations, but as of right now June 5th 6PM, there is. 
You can actually add the vehicles to one location, at one particular set of coordinates, and then jump in to a different vehicle and drive 10 feet away, and add that vehicle to the same group, both vehicles will now spawn at either coordinates within that location.
Feature being worked on right now, :
Only the first vehicle shows up on the radar and only the first coordinates are displayed, any additional coordinates aren't shown in the INI, this not a bug, but due to the code not being complete.
You can currently see a prototype INI file, in which 3 Vehicles are assigned to a group. Either vehicle may spawn at either coordinate.
This modification is a work in progress and is in it's very early ALPHA stages. There is A LOT to be done still. AHK1221 has worked VERY HARD on this modification.
Finally, this is not a portrayal of the final product, but a concept. However, since this Modification was inspired by me and is being brought to life courtesy of AHK1221, and those who have provided support to him. I figured as an alpha-stage tester of this mod, I would give you a sneak peak of our progress so far. I'm thinking that we need to come up with Categories and then put the groups inside of the categories as well, based off the way the script currently works. However, it may be a lot of work. We'll see what AHK1221 has to say about it when he wakes up. Please feel free to provide any suggestions or feedback as to how the mod should work, how it currently works, and what you think so far, what changes may need to be made. If you're willing to support AHK1221 by becoming his patreon, he will very gladly appreciate it. https://www.patreon.com/user?u=5743056
AVS 0.2.2 Version INI Format
Name=Shipyard M
Simultaneous_Spawning=True
;Spawn Coordinates
[L1Location]
Actual_Name=L1Location
Display_Name=L1
Position=X:1135.962 Y:-3227.891 Z:5.509732
Rotation=X:0.1123545 Y:-0.03912427 Z:-0.5506954
Enable_Blip=True
Blip_Color=Yellow
[L2Location]
Actual_Name=L2Location
Display_Name=L2
Position=X:1131.691 Y:-3227.744 Z:5.510054
Rotation=X:0.145867 Y:-0.02439503 Z:-0.7233396
Enable_Blip=False
Blip_Color=Yellow
[L3Location]
Actual_Name=L3Location
Display_Name=L3
Position=X:1127.744 Y:-3228.092 Z:5.504296
Rotation=X:-0.3936107 Y:-0.01072656 Z:-0.6011391
Enable_Blip=False
Blip_Color=Yellow
[L4Location]
Actual_Name=L4Location
Display_Name=L4
Position=X:1123.748 Y:-3227.768 Z:5.482669
Rotation=X:0.5260214 Y:-0.03617152 Z:0.7445337
Enable_Blip=True
Blip_Color=Yellow
;Vehicles
[Ferrari FXX-K Hybrid Hypercar]
Engine_Running=False
Hazard_Lights=False
Lights_On=False
Vehicle_Name=Ferrari FXX-K Hybrid Hypercar
Model_Hash=234062309
Locked=True
Alarms=True
L1Probability=0
L1SpawnRate=0
L2Probability=100
L2SpawnRate=100
L3Probability=0
L3SpawnRate=0
L4Probability=0
L4SpawnRate=0
[2015 Porsche 918 Spyder]
Engine_Running=False
Hazard_Lights=False
Lights_On=False
Vehicle_Name=2015 Porsche 918 Spyder
Model_Hash=1588380823
Locked=True
Alarms=True
L1Probability=100
L1SpawnRate=100
L2Probability=0
L2SpawnRate=0
L3Probability=0
L3SpawnRate=0
L4Probability=0
L4SpawnRate=0
[2007 Lamborghini Reventon]
Engine_Running=False
Hazard_Lights=False
Lights_On=False
Vehicle_Name=2007 Lamborghini Reventon
Model_Hash=418536135
Locked=True
Alarms=True
L1Probability=0
L1SpawnRate=0
L2Probability=0
L2SpawnRate=0
L3Probability=100
L3SpawnRate=100
L4Probability=0
L4SpawnRate=0
[2014 McLaren P1]
Engine_Running=False
Hazard_Lights=False
Lights_On=False
Vehicle_Name=2014 McLaren P1
Model_Hash=-129939002
Locked=True
Alarms=True
L1Probability=0
L1SpawnRate=0
L2Probability=0
L2SpawnRate=0
L3Probability=0
L3SpawnRate=0
L4Probability=100
L4SpawnRate=100
Outdated INI Concept during planning stages:
0.0.0 Alpha Stage Concept
Outdated INI file Concept
[GROUP]
Name=Military HMMWV
[Military Base]
Location_Name=Military Base
Display_Name=Military Base
[Spawn Coordinates]
Position=X:274.0487 Y:1076.341 Z:224.4177 // Near South Hangar A1 (Unspecified Exact Location)
Rotation=X:-0.9772298 Y:-5.530599 Z:126.6914 // Near South Hangar A1 (Unspecified Exact Location)
Position=X:226.0487 Y:1376.341 Z:224.4177 // Building B3 (West Side)
Rotation=X:-0.9772298 Y:-5.530599 Z:126.6914 // Building B3 (West Side)
Position=X:254.0487 Y:1626.341 Z:224.4177 // Building B2 (South Side)
Rotation=X:-0.9772298 Y:-5.530599 Z:126.6914 // Building B2 (South Side)
[-1041692462]
Vehicle_Name=M1025 4x4 High Mobility Multipurpose Wheeled Vehicle (HMMWV) UA Basic
Display_Name=M1025 (HMMWV) Humvee
Model_Name=-1041692462
Probability=90
[418536135]
Vehicle_Name=M1043 4x4 High Mobility Multipurpose Wheeled Vehicle (HMMWV) UA Special Forces
Display_Name=M1025 (HMMWV) Humvee
Model_Name=418536135
Probability=75
[998262399]
Vehicle_Name=M1114 4x4 High Mobility Multipurpose Wheeled Vehicle (HMMWV) UPA Tactical
Model_Name=998262399
Probability=95