Door and cabinet hardware - in my previous house we had a couple of doorknobs/levers that needed to be replaced. Emtek still carried the style, but the detailing was just a bit different and the color was slightly different. Not that the average visitor would ever notice the difference!

Other than flooring not sure i’d do much else. pretty sure whatever you buy extra of is not going to break and something else will. at least thats the way my life works. and if something breaks 20 years down the line chances are you may want something different anyway…

Well I can almost guarantee I will never remodel another bathroom in my life :)

I did buy an extra one of my flush mount light fixtures with a square glass shade. My husband asked me if I had an extra shade so I did admit it. I was not planning to tell him as I figured he might be more careful if he thought we would need new fixtures if one broke.

I kept a few bundles of heavy cedar shakes after reroofing. They made my shed smell fresh and did come in handy after a rooftop solar project damaged a few. A shake roof is good for 50 years or more in my climate but they are hard to find and I don't expect that to improve.

Extra bits of flooring and tile are also useful to keep.

I do not have space to store an extra toilet. Perhaps I would if I didn't have all the flooring and roofing materials. 😉

I am thinking the toilet might be a bit extreme, but on the other hand, I always find it a bit disappointing when I see colored bathroom suites in real estate listings and they had to replace the toilet with a white one. Not the end of the world of course,

I didn’t buy the whole light fixture but I did buy multiple extras of the glass shades on a new mounted on the mirror light fixture we installed during a master bath remodel. The shades needed to be set just so into the bracket on the long bar of lights. Sort of an art deco style. We learned, purely by chance, that the light we bought was a discontinued model. I had visons of dropping one of the shades when changing a bulb or cleaning it. Not only did I use a couple of the replacements but about 6 months after we sold that house, our buyer reached out to us through our realtor asking for info on that light fixture. They had broken a shade & were looking for a replacement. They had forgotten about the box we left under the sink!

What about the stopper or faucet for the sink?

Actually one the posts for the pop up drain did break at the interface between the knob and the post. Stupid. With the quality of things these days you could need a spare of almost anything.

Extra cartridges for shower faucet - DH just replaced ours yesterday and said this particular one has been discontinued by the mfg. He has a stash :-)

Oven / cooktop knobs, heating element, and some electronics if you can.

I have extra light bulbs for my odd fluorescent light fixtures in the bathroom. I did a really simple cabinet door pull in the bathroom. The two drawers have push-to-open drawer glides. I read that those tend to break, so I have one extra. But I also bought the door pull for both drawers, just in case they all broke!

Berenson 1065 Bravo 9-1/16"L Finger Tab Large Cabinet Lip Pull / - Polished · More Info

Well, I am installing a range top style cooktop, and I am having them make a matching drawer front and rail to fill this in, in case I switch to convection down the line, so I don't have to redo cabinetry with a switch of appliances.

I buy fabric of anything I get upholstered. I just recovered my kitchen chair seats and bought extra fabric in case a stain happens that just won’t come out

Paint samples, especially for anything custom mixed or if a painter claims that they color match. I had to do some touch ups in a room with a fairly dramatic color, and the sheen and color were never quite right.

cabinet door/drawer pulls. When I got new cabinet doors and painted our kitchen, I replaced all of the door/drawer pulls. When we decided to later add an island, I got door/drawer pulls ordered right away, even though it took an additional 5 years to actually order the doors/drawers.

All toilet seats are designed to fit one of two bowls: round or elongated. No need for a backup, unless, as you say, the fixtures have a unique, trendy color and which could be discontinued.

Toilets come in gray??!!

Of course it’s your home but I personally wouldn’t install fixtures in any color but white or off-white.

I have strong memories of a spare bathroom in a former home with a harvest gold toilet, bathtub and sink. All in perfect condition and therefore didn’t need replacing, according to DH. 🙄

I am not a fan of harvest gold bath fixtures, but I would side with your husband on that one: no need to spend $25-50K to replace older fixtures in perfect condition, triggering an entire remodel. Especially since they were probably manufactured to a higher finish standard than today's fixtures.