Although trying to make the vocal have a wide stereo width is not a bad idea, recording a vocal in stereo is very rarely done since you are dealing with a mono source to begin with. You want the vocalist to have that up front presence, so you want them close to the microphone. Even if you had two microphones very close to try to record in stereo, it would still be a very mono sound.
Likewise, using a stereo expander on a mono vocal track isn't going to do anything for you since there is no stereo image to expand upon.
A better way to achieve a wide stereo sound for a vocal track is to use some sort of subtle (or not so subtle) flange or chorus effect. Listen to the second Seal album to hear some not so subtle flange on a vocal. You don't want it to be a deep flange that has the typical flange sweep to it, and you want the rate to be very slow, almost static. Since you are dealing with a mono track, you don't want to do this as an insert, because the result would be mono anyway if inserted on a mono track, plus you want to keep the dry signal as well. Set up the flange or chorus as a send effect and adjust the blend to taste.
Other methods of achieving a wider vocal sound would be to set up a stereo pitch shift that you again use as a send effect (not an insert). Pitch one side down about 10 cents or so and the other side up about 10 cents or so. If you can put a small amount of delay before each side (somewhere in the 20ms to 50ms range, but different for each side) that will make the effect even more pronounced.
As far as other vocal effects go, big reverbs were really a thing of the 80s, and you don't hear tons of reverb on vocals anymore, except maybe for slower ballad type songs. Typically, shorter, more natural reverbs are used when you want to put the vocalist in a space, but again only use the reverbs as send effects, not inserts, so that you can still keep the dry vocal very upfront. The reverb should be more in the back and you really need to tweak the parameters to make it blend in and sound natural. Also, putting a small pre-delay on the reverb will help keep it separated from the dry vocal a bit to make the dry voice stand out more.
More often these days, delay is used to give the vocals some space. The delay can sometimes be obvious so that you hear the echoes, but often it's treated more like a reverb, where it is further back and set to just give the vocal a little space without being heard as discrete echos. Another favorite trick is to then feed the delay into some reverb... this helps to wash out the echoes a bit and gives more space to the vocals without have to apply reverb directly to the vocal.
However, the most common processing done on a vocal in today's pop music is compression, and LOTs of it!! If you want that big, present, in your face sound, you can't be afraid of compression and limiting! First, though, you've got to have a good raw track to begin with! Picking the right microphone for the vocalist and setting it up right is key! You need to capture a sound that already has a lot of presence to it. Usually this means getting the vocalist pretty close to the mic, and choosing a microphone that is fairly bright, without being too harsh. A good large diaphragm condenser microphone with a built-in presence boost to it is the key to this sound.
You'll want a good outboard analog compressor/limiter to compress a bit on the way in. Be careful with your settings on the way in. Don't overuse the compressor on the way in unless you really know what you are doing, because you can't undo it afterwards. The compressor on the way in should mostly be to allow you to get a hotter level to your A/D converters and protect you against clipping the converters. However, a really good analog compressor can be part of the sound as well. Many compressors in the big studios are used mainly because of the sound they impart to a track, not just the compression abilities.
During mixdown, you'll want to compress and limit much more using the best sounding compressors you can get your hands on, and often more than one compressor/limiter. In the big studios, it is not uncommon for the mix engineer to chain together 2 or more compressor limiters for the lead vocal. The first one might be set to just gently smooth things out a bit overall. The second one will be set a bit more aggressive to smooth out the big peaks that get past the first compressor and to level everything out even more. After that you might set a third unit to be a heavy limiter to capture and squash any big peaks that make it through the first two.
Another trick is to set up different compressors for different parts of the song. If the person is singly softly during the verses, and there is much more space for the vocals in the verses (mellow background music tracks), you will probably want to use much more gentle compression to just smooth things out a bit and so that the vocal has a little more relaxed feel to it. Then, if you kick into a big chorus or bridge section where the vocalist is singing much harder and the backing tracks are big, then you'll want to do some very aggressive compressing and limiting so that the vocal can be very loud and even and has an aggressive quality to it. Using computer DAWs this is quite easy to do as you can just separate the verse and chorus vocals to different tracks and insert compressors/limiters with different settings for each part of the song.
When you start doing a lot of compressing/limiting during the mix, you'll now see why it is important to have a present and bright sounding vocal track to begin with. Compressing/Limiting will always tend to make things a little less bright as it clamps down the attack transients of a track. The more you compress and limit, the more this will affect the sound, especially if you are using very fast attack settings on your compressors. You may even find that you'll need to use some EQ or an exciter after the compressors to bring back some of the high end that was lost.
Doubling is the big thing if you are looking for a thicker sound. But, the rapper has to be very accurate for doubling to work right.
One of the things we do quite often is to double up the hype tracks and then pan them hard left and right. The lead track is still single, but when the hype happens, it really fills things out.
Also, I tend to use a subtle chorus or flange on many rappers who need thickening. Nothing obvious, just enough to fill things out a bit.
You might also try a short room reverb to add just a touch of ambiance, but keeping it very low in the mix. Just to make it sound a bit more natural.
Also, you usually want that big, in your face sound, so I get the rappers up very close to the microphone to take advantage of the proximity effect (doesn't work if you are using an omni-directional mic). Make sure you use a pop filter when you have them that close.
Hopefully these suggestions will get you started experimenting and finding a sound that works for you! There are no rules, so trust your ears and do what sounds good!