re: garbage.
Isn't this already achievable with current pakset factory dynamics? Just set waste to be an output item for any factory and voila, you've created a waste stream. Create recycling depots that are urban and have a set percentage of waste received turned back in to end products... steel, goods, etc. Of course, this creates a bit of a circular loop/recursive situation where an end factory feeds goods back in to itself via the recycling depot... not sure how Simutrans would handle this or whether it would care. Might be better to handle that as waste being an end delivery factory (landfill) and leave it at that.
Setting all buildings or commercial buildings to produce garbage probably isn't a good idea... as mentioned, mail is already onerous enough if you choose to fully service a city. With a limitation of 1 road vehicle per tile, you'd badly overload your roads with having to add garbage trucks to the mix.
That said, being in the solid waste industry myself (I run a business that operates landfills) I'd be quite keen to add that element to my games

re: 1st class and 2nd class.
1st class passengers are just a subset of 2nd class passengers. 5-10% or whatever of passengers are willing to pay for the extra comfort and conveniences of 1st class but still travel on the same vehicle to the same destinations. The game has already modelled this in, it just isn't split out and apparent. When you fill your Airbus with 250 passengers, 25 or whatever of those are already 1st class. When you receive $x for your passengers for the flight, it's really $y for 2nd class + $z for 1st class = $x for the plane load of passengers. To keep the same financial balance that the game/pakset has now, you'd lower the fare for 2nd class a bit and increase the 1st class fare... and you'd still receive the same revenue as you do now, otherwise you'd imbalance the profit/expense ratio that the paksets have created. If you create a private business jet for your network and fill it up with regular passengers... those are really 1st class passengers (even more 1st class than 1st class!) who are willing to pay a much higher fare for the comfort of private jet travel. And, of course, expenses are equally high per passenger... and profit/passenger isn't that much different than any other form of travel.
I think adding 1st class passengers isn't really necessary... it's mostly a cosmetic thing that doesn't offer much to the game other than adding complexity for a new player. The comfort level in Experimental is more than sufficient in my view.