How would you go about really trying to fully immerse the player in the game? Would love to hear some suggestions on how y'all would go about doing this, willing to listen to anything.

Great, well-thought-out sound design and top-notch art.

Marth8880 wrote:sound design

I second that.

Detailed environments. A lot of forethought. Not just placing props "willy nilly".

Something I believe adds to an immersive gamepaly experience is well thought out units and balanced gameplay elements. Its very important that a unit is given a lot of thought to incorporate unique attributes. Having a lot of units can be nice, but sometimes you just have to limit yourself and ask does this unit really sepreate itself from other units? Or do I really need 100 of these units with a slightly different rifle? Unique appearences also aid in immersion. For example a unit that utilizes heavy weaponry should have some kind of part/parts that help indicate he is a heavy type unit. Things like thermals on his belt, backpacks carrying ammunition or extra bits of armor can help it feel more specialized.

It doesnt hurt to make nice looking maps that make the player want to change his strategy's up. Geoemtry and geology are something that should be unique for each map. Adding subtle things like crates or props in reasonable areas can help make a map. Also just like the unit suggestion, it doesnt have to be a huge map. Huge maps take more uneeded space like large rosters of characters do. If a map doesn't need to be big, dont make it big. Dont be ashemed to use vanilla assets either, with some clever -retexturing you can literally make different assets from different maps fit together in one coherent theme. Alzoc Blizzard is testomony to that notion using mos eisley, polis massa and several other assets and giveing them a nice winter theme reskin and making it feel like its a new map alltogether.

IMHO:
Consistency in tone.

In about every type of game you could think of - be it boardgame, cardgame, whatev-genre videogame - you'll find games that have been extremely immersive for some/a lot of its players. The way this was most consistently accomplished was to have an initial "thing" (mechanic, atmosphere, genre, premise) that would be interesting to play with/within. Then you hone in on that target group which you think will connect best with that initial "thing". These become the main 2 elements of your design later on, keep them in mind. Then when you design/build your game around that initial core, you should question wether or not each and every element you ad to the game supports what the "initial thing" was about and if it supports what your "designated target group" would feel comfortable with. The shortform way of describing this is tone; ie "is everything in line with the feeling we want to deliver, or is any element detracting from that experience?". This is why there are Star Wars games that have really connected with people. For example, SWBF as its basic premise is experiencing a battlefield in the fiction that we love so much and most of the elements in the game are there to support this idea within the constraints of computer space and the time they had to work on the game. Visuals, stats, narrative delivery and tone, cinematics with Darth Vader in them, etc...
But then there are also RPG's in many flavours that are connecting with many different people. Hardcore numbercrunching RPG's because there's crossover group of (potentially hidden) math "geeks" (in a solely positive context) and Fantasy aesthetic lovers which then makes it possible to build immersive worlds around these factors.

This is my short, very to-the-point thoughts on this where I'm skipping over a lot of reasoning. If you want to get into Game Design, a great place to start is the book "Game Design - A Book of Lenses" by Jesse Schell (ex-Disney imagineer, now Game Design teacher). If you're interested in the book, pls pls pls, buy a physical copy of it. This physicality becomes very important later on.

Now it's time for me to get to work. My internship coach is eyeing me suspiciously. Have a great day! I hope you find this answer somewhat relevant to your question.