If I don't ask, I'll never know

A. One big custom map containing all of the main buildings (with generic buildings filling the gaps)
B. Replace purchaseable outfits with purchaseable characters (Homer, Bart, Marge, Lisa, Maggie, Apu, Moe, etc...)
C. One Gil placed somewhere on the map, with all vehicles to purchase (Including Modded Vehicles: Homer's Sedan with Roof, Marge's Station Wagon, Thrilhouse, etc...)
D. A custom campaign that has a more classic Simpsons feel to it, like seasons 1 - 10 (With missions that require you to purchase other characters and vehicles in order to play)

And before people say that it's not possible to combine the three maps together... That's not what I'm asking. I've seen the trailer for the Donut Mod 4, that map is entirely custom built, so could a custom map be made with all the noteworthy buildings (Simpsons House, Kwik-E-Mart, Church, School, Krusty Burger, Lard Lad's, Powerplant, Moe's Tavern, Town Square, Town Hall, Sit & Rotate, Comic Book Store, Planet Hype... You get the point) from the other three maps?

That sounds pretty good

Until Donut Team releases the Map Builder, such a mod is not possible, no.

One big custom map - doable, you can already start sketching it up in sketchup
Replace purchaseable outfits - doable, but there's a limit on costumes, and I don't remember how far you can go
One Gil with all vehicles - afaik, there is a limit too, since it's a shop aswell, and I don't really think that's doable
A custom campaign - surely doable

It would be better to wait until the map builder to come out (which won't be for a long time) to do 'A'

'C' is pretty pointless since there is already a hack to unlock all vehicles in the mod launcher without collecting the money in the game.

I would recommend either 'B' or 'D'

(Also, I wouldn't put this in the 'rant' category. Try 'help needed' instead)

What is Sketchup? And how do I find it?

@Freelancer_T_O_T Here is SketchUp

You probably want SketchUp Make: www.sketchup.com/download/make