Before you ask for any of these ideas "Why not put it in general discussions? They might add it!", yeah, I'm unsure if any of these will ACTUALLY be implemented or added. If you want to attempt at making these mods, go ahead.

1. Passenger Interests (First difficult idea)
Passengers will have specific interests to different vehicle types, as to simulate people's preferences on travel. Example: Certain passengers will want to take a plane rather than a boat as to simulate sea sickness, vice versa. This doesn't mean passengers WON'T take a specific type of travel, it just means that if available and accessible, they will take it. This doesn't go for things such as close proximity travel. (EX: Bus 'shuttle' to Airport for a Plane. They prefer planes but will still take the bus.)

2. Ticket prices / Freight Costs
Tickets can be adjusted to be more expensive or cheaper. This will affect your revenue but will affect how many passengers may board depending on it. If there's a cheaper alternative passengers will take it. If there is no cheaper alternative, passengers could pay the price, but there's a chance they may not want to travel, settling for private transport.
Freight costs mean that you can adjust how much you sell freight loads to workshops or how much value a unit of cargo has. If it's a cheaper cost, there can be an increase of manufacturing or greater demand. If it's more expensive, there's a chance manufacturing will decrease, or there will be a decrease in demand.

3. Comfortability, speed, and Destination.
Basically, Comfortability will rely on your vehicle's age, condition, and build date. For example, if an old carriage is just built, everything will be fine, passengers will be comfortable. However, as more carriages are made, passengers will lose their comfort on the carriage. This means your revenue will be decreased, or passengers may stop riding. This also balances with it's age in just bought to ++ years. The more aged, the less comfort it may have. Condition just means the more worn out it is, the less comfort it will have for passengers.
Speed means that if the travel is faster, revenue will be more. Think of it as 'Express and Local'. You're willing to pay more for an Express ride because it gets you to your destination faster.
Destination means that passengers will seek the fastest way to get to their destination, repeatedly swapping to other routes possible. This means transfers are more likely, and this can be implemented to things like Bus > Plane > Bus rather than a long bus / train ride purely because it's faster.

4. Depots, Yards, and other parts. (Probably difficult)
This is just as a way to conserve vehicles. If a vehicle's condition is getting worn down, you can send it to a yard/depot and have them conserve in the yard. It can be like a station or stop, or hangar, where they stop there until you give them a route. During this, you can repair their condition and make them brand new, with the cost of maintenance.

That's all I have. If there's any mod ideas you have that kind of follow this layout feel free to post them here, and if you think you can CREATE these mods, you are free to attempt to do so!