The approach you might want to take is not just to make a new genre of game modes, but add on to the ones that are present. Here is a list of a couple of examples:
- Making an expansion of trucking server; Add more delivery-type jobs by train, boat, plane, etc. Also add in different items. Maybe some things to buy with the rewards you receive like houses.
- Making an expansion of racing server; Add special races (or base the whole theme on) with upgrades or wild things. Such as objects across the road or pickups that will lower/increase your car damage. You could even make something relatively like Mario Kart (like Grim_ posted before)
Also, roleplay wouldn't be all out of the water. Don't make your basic roleplay, but make it have a central theme. Instead of just going out, getting a job, joining a faction, buying a car and house and living it big, make it based on something.
For example, you could have a race game RPG. You start out with a crappy car and have to earn cash by winning street races or doing jobs around town. You progress by winning races. With the extra cash, you can buy new cars, and equipment. As you get better, win more races, your name starts to get heard and you unlock more races. Vary it up, have checkpoint lead races, Point A to Point B races, etc. You could also expand the trucking server and have it based on the central idea of becoming the greatest shipper. And if your shipping abilities are heard, you could ship things with gang members, like cocaine or stolen cars.
A thing I always remember when planning a gamemode: One with a theme will always be more structured, have better game flow, and be enjoyed more.
Suggestions
A gamemode that I was creating that was based on a central idea; Auction. You would start out with three items: CJ skin, brass knuckles and a bike. Everything is saved, and buy-able, including:
- Skins
- Vehicles
- Weapons
- Fighting Styles
- Colors (Scoreboard/TAB colors)
- Few objects (Only a few for the sake of the server stability)
- Few pickups (Only a few for the sake of the server stability)
- Items (drugs, beer, medical kits, and other things related to the auction/trade system)
Some items would be buy-able in the stores, some would be auction only (random auctions generated by the server. The chances of it being an auction only item would be 3/100). Others being premium only, where only donators would receive them. Players could start their own auctions as well, for a set price, and for how long they want (with a max & min set). Price to start on the auction would be a ratio between starting price, length of auction and player wealth (like .5-3%). There would also be a trader's island where you could trade items amongst other players.
You would earn money buy dming, racing, auctioning off items, finding secret items, playing mini-games, or winning campaigns. Campaigns would be special events, which would include dm, racing, ... which would start every X minutes. The only difference is you would need to own the item of the campaign to participate in it. For example, if it was a race with infernus. You would have to own the infernus in order to participate in the campaign, otherwise, it would reject you from participating.
There is loads more I can share about the gamemode I had planned, and actually started to, create. If you would like some more information and ideas that came from it, shoot me a PM. I'd be glad to feed you ideas about this mode, as I would love to play something similar to it. Just don't have the time to make it myself, unfortunately.
ur turn 
