Agree about the glass shower and also agree with avoiding custom. I think I paid $2500 for the glass twenty years ago, and it is only two and a half panels of glass. I don’t think HD was selling stock glass showers twenty years ago.
I did the boys’ bathroom as all white and I think you can liven it up with paint color on the walls, towels, and a rug. Given the size of the house, I am not expecting a palatial master.
I just remodeled two bathrooms.
One was in white subway tile, with a Toto tub, shower, and shower curtain.
The other was a shower only, with Kohler glass doors, that I purchased at Home Depot for $350.
The tile in that bathroom was porcelain, that looked like carrara marble. I had a seat built into the corner.
Unlike most, I am actually a person who prefers baths. I have an old clawfoot tub that is wonderful for soaking, because it is deep and narrowish. The absolutely worst thing for a bath lover is a shower over tub with one of those shallowish tubs with non-skid stuff on the bottom. Presented with that situation, I take a shower. I never take a bath in a hotel. Jacuzzis are okay, but too many of them are so huge that it is a terrible waste of water for one person.
If and when we add a bathroom upstairs, It will have a really nice large shower and no tub. I already have the tub. 
If I were adding another tub, it would be similar to the one I have now.
I think that a detachable shower head is a necessity in a shower these days. One, if there are multiple heads.
I’m all for white tile bathrooms. You can add color with paint and towels. Colored vanities are okay, though.
I’ve got white tile in my master bath but the walls are painted a periwinkle blue and the sink top is this one on a light wood vanity: http://www.faucetdepot.com/prod/Pegasus-PBI31W-Modern-31%22-Glass-Vanity-Top-With-Integral-Bowl-in-White-87036.asp (except mine s 48" wide). So plenty of color in a largely white bath. I agree - I love having a detachable shower head - it’s my only shower head in fact.
“The absolutely worst thing for a bath lover is a shower over tub with one of those shallowish tubs with non-skid stuff on the bottom.” Amen to this, @Consolation. Agree with the rest of your post 100%. I’m an occasional but an enthusiastic bath lover. Showers 95% of the time. But that 5% bath time is mighty important.
Funny, I adore a bath, but I hate my big master jacuzzi tub, I like the old small tub with a door I can close to keep it all steamy! I am tempted to take out my big spider/dust catcher tub and do my own big shower
I’ve got one of those awful two person jacuzzi tubs in my bath. It takes so long to fill and is such a waste of water that I never use it, except as place to keep an extra clothes drying rack. I’m going to update the master bath before I put the house on the market and I’m thinking of replacing it with a soaking tub. I’d love to put a soaking tub in the house we’re remodeling, but it’s an older home with small bathrooms.
We tore out the tub/shower in the “new” house master bath and put in a larger shower. I used light gray Eramosa porcelain tiles. I like the way it turned out though it may be a bit trendy.
In the second bath, I used 4x12 white ceramic tiles with an undulated surface paired with a marble mosiac.
http://akdo.com/product/eternity-imperial-carrara-h-w-thassos-h/
I was nervous about the combination, but I love the way it turned out. I hope I don’t regret the mosiac - marble and lots of grout. Sometimes I don’t make the most practical decorating choices.
Check with local realtors for the wishes of the market for your size/location. Some love a tub, others think they want one even if they end up not using it. Be practical. A deal breaker could be the money a potential buyer figures they need to spend to get rid of the trendy, bold, interesting…
Check at the local Home Depot, Lowes and tile places for what is being sold at your price point. Having a nonleaking, functional shower without changing the size may be all you should do. I’ve been known to state “the next owner can…” for things that could be changed but are easy enough to do so if that twenty years from now new owner want to.
Find out what sold in the nearby homes- presumably they are of similar size and quality.
I would skip the fancy marble and go with less expensive materials with lower maintenance. We reused our obscure shower doors when we remodeled our old/new to us house a few years ago. Don’t spend more for features others may not want.
In terms of remodeling for sale, I think the suggestions given by @BerneseMtnMom are excellent. You don’t want to spend too much money on something the new owners may rip out. On the other hand,
many homebuyers are put off by outdated bathrooms so having an updated, clean look helps sell the house. I will likely incorporate many of those suggestions when remodeling the master bath in the house we’re selling.
Contrary to many here, I LOVE my huge spa tub. When we redid the bathroom we replaced it with one that has jets with colors, heat, etc. It holds two people comfortably :). We dod add a longer bench in the shower rather than a little moon shaped seat thing in the corner. I do recommend this. We have glass, but many new showers are flat with the tile floor and no door.
make sure whoever does the work installs a shower pan to code (and do not try to save by skipping inspections) a poorly installed shower pan and or just keeping the 30 -40 year old one to save money just leads to massive expensive headaches when the leaks start. (murphy’s law guarantees you will have leaks if you do it the “cheap” way)
I am getting price on the tub out, shower in costs. If I do that, I still have that 3x3 +/- shower to deal with, re-purpose into a linen closet? Something else?
The houzz site has great pictures and ideas. If you’re doing it to flip, I would work at containing the costs. For example, tile around tub, but not the whole room. If you purchase a vanity for a single sink, you will probably be able to get a remnant at a quarry/stone purveyor for the top which will cost a fraction of what a whole slab cut to size would cost for a full length counter. And yes, accessibility is appealing to most buyers.
It would be about $5k more in labor to do that change to a new shower.