Personally, all of your ideas would definitely make World better than it is. However, some ideas can be built upon! The thing with the engine swap, my only issue is what would be the point of doing an engine swap if you can't really tell what the engine is. In other words, we should also have a way to open the hood, like in UG2. But the basic idea of putting a modern Coyote engine in a 69 Boss is... mind blowing!
Anyway, the tier changing, not a bad idea, but I like making people guess how car performs (but still getting it wrong after I make em eat dust in a race
). Basically, it'd turn cars into sleepers if they still had their original tier, but the power of the next tier up (like the Civic you mentioned).
Music... not much more to add to that. The idea of allowing the game to use your personal music files sounds good though (too bad it'd take up memory). I mean, I have a folder made just for this game: Racing (it has songs from previous NFS games, and other racing game). I play it every time I feel things are too quiet in the game.
Now, my personal favorite part of the game, and what makes NFSW a true social game: vinyls. Yeah the vinyl designs from UG, UG2, MW, they were col and all. But like Dr Houbenheimer said, "premade vinyls such as the unique vinyls from UG2 would mean that your gonna have a lot of people driving around in the same design". The main issue we have with vinyls is the vinyl limit. 31 vinyls just isn't enough for designers like me. Look at my profile, you'll see some of the cars could use a few other things added to them. Anyway, the first priority should be making the vinyl limit 50, adding new styles is second to that.
And then there's custom bodykits. That has to be the biggest failure of NFSW, having set bodykits! I thought this game came AFTER NFS Undercover! Undercover, Prostreet, and Carbon had custom bodykits, AND Autosculpt! Why did we all of a sudden go back to Most Wanted? Even Undercover had set bodykits, but they also had the ability to make custom bodykits! I mean, do you really wanna see a Stingray or Charger R/T with a spoiler? Doesn't look good on a classic like those! Custom lights will be on the back burner (does a set of custom taillights look good on a 69 Charger?)