It's not even like there isn't room for more.
They could easily expand the amount of Raccoon Downtown by introducing more areas after the first Nemesis fight at the Construction Site. I mean, all that happens in this area is Jill seeing Kendo, going through a house, and then Nemesis shows up with his Rocket Launcher. This area could be a second Downtown area complete with missing areas from the original Downtown - News Office, Restaurants, Shopping Centers, maybe even areas we see in Outbkreak, etc. It felt a little rushed to fight Nemesis only to have him be there again RIGHT away with his Rocket Launcher.
Additionally, after the train crash - you go right in to the second major Nemesis boss fight. But it is confirmed that this is a public park area and the Clock Tower is right behind Jill. They could easily open up these areas to make them more accessible. They could include a few more Hunter Gammas in the park and even include more Drain Deimos in these areas allowing more variability in enemy types.
Every time I went through the game, I plotted ways to introduce new and better areas to it.
Excellent ideas, you've inspired me to do a rewrite of the game similar to when I did a rewrite of Resident Evil Zero. The reason being like you said, there was so much they easily could have done with RE3R to turn it into a more respectable remake. Actually if you look at the concept art you'll see that there were lots of levels that ultimately ended up on the cutting room floor. Moon's Donuts was originally supposed to be a its own separate building located in a much more open area, and there was a bar level that's nowhere to be seen in the final product.
The events you mentioned would've been perfect opportunities to extend the game's length. Especially the second one, since I feel like they wanted Jill to hurry back to the subway as fast as possible after the rooftop fight. That's kind of a recurring problem with this remake's whole structure though, the developers wanted to evoke a sense of non-stop adrenaline rush where the escape from Raccoon City constantly feels like a ticking bomb. That was a mistake from their part, it feels like they were more interested in retelling the game's story from a more cinematic and exciting viewpoint rather than respecting the original. Raccoon City wasn't a ticking bomb until October 1 when the strike was ordered. Up until then, I think a more slow-paced escape from Nemesis would've worked just fine in this modern remake just like in the original.
Here's how I would've done it:
In order to avoid the need for Jill to hurry back to the subway after the rooftop fight, let's change the subway story plot so that civilians are being lined up in greater numbers, and the train has to go back and forth a few times. The story can still play out kind of the same way, that Jill lures Nemesis away from the subway in order to buy time for the civilians being rescued. But instead of just being one single train of civilians, lets make it so that they have to go back and forth at least once, therefore giving Jill more time to toy around with Nemesis before heading back. I think this would be a great opportunity to include Jill's RPD part. After the sewers and the rooftop fight, Jill still heads to Kendo as before, but Kendo gives Jill some new information: that a wounded Brad was there just earlier and headed to the RPD to look for her. "Brad's alive? I've gotta find him!" This gives Jill a reason to enter the RPD, to find Brad and take him to the subway. After Kendo rejects Jill's offer to get out of the city, she asks for the spare ID card she entrusted to him - she then uses this to head into the RPD via the parking garage.
In the basement of the RPD, Jill encounters a few cerberus and lickers, and has to solve an electronic puzzle to activate the elevator to reach the 1st floor. At the first floor, she runs into Marvin and a few other police officers and helps them fight off a few zombies, she tells them about the subway plan but Marvin says that Jill shouldn't trust the UBCS. Afterwards, Jill goes to find Brad hiding in the STARS office, and Brad is very frightened at this point. He says that he's afraid of the Nemesis, but Jill tells him to get up on his feet and come with her, that "they've found a way out of the city". At this point it would be fitting for Brad to say something like "You don't understand, Jill, there's no way out of here with that monster still alive chasing our asses, the only thing to do is.....". Then, they hear a loud bang, followed by heavy footsteps that come closer and closer..... The tension rises, and then Brad says "It's him. He's found us." They quickly run out of the Stars office and make their way through the corridor only to have Nemesis appear in a cutscene.This will be Brad's death scene, as Nemesis grapples his with his tentacle, drags him towards him and then shoves a tentacle right through head just like in the original. With nowhere else to run, Jill heads north to the shower room and pulls up towards the wall. Just as Nemesis is about to smash right through her, she makes a dodge and Nemesis smashes right through the wall instead (tie-in to RE2). With Nemesis a bit fuzzy from the smash, Jill takes her chance and starts running - this will be a chase sequence and you can't down Nemesis in this part, you have to run through the RPD and escape before he gets you. The problem is that the policemen have put up new barricades since before, so t will be kind of like a tricky maze to find your way out. The only way out is back to where you came from - the parking garage. For example, if you try and make it out to the front gate Nemesis will appear at the outside and make it impossible for you to escape that way. Once down at the parking garage, Jill makes it back to Kendo and knocks on his door, but he refuses to come out. He tells her to grab the key on the wall and then the next plays out the same way as in the actual game.