I'll start with my general essentials - and then I'll go back and grab the tips from other seasoned travelers!
I have 2 washbags for travel. I use Muji bottles, and I paint on a letter using different coloured nail varnishes to the lid for contents. So in my "longer stay", I have 100ml bottles for shampoo, conditioner, shower gel etc, and use 50ml bottles for overnights/2 nights, both of which I top up when I return home from my standard bottles in the bathroom. Always top up on return as it takes only minutes but saves a LOT of stress when packing again. My favourite toothpaste doesn't come in travelsize packs, so I have a muji tube which I fill with toothpaste (and MUST keep separate from DH's washbag, as he has a similar tube with moisturizer! He is not so bothered using colgate when travelling). As everything else is reasonably controlled, I take a full sized deodorant (100ml).
I have a small (5/10ml?) spray dispenser for perfume that I got in an airport or chemist somewhere, you fill your regular perfume from the pump dispenser in the base and it is soo handy.
And when I am buying my regular skincare (I went through a phase of Estee Lauder and have to reign that in again), I was always going to the same counter of the same shop to buy them, and asking for a sample when I went too so get the same in the small sizes and labelled (when you chat to them, they generally find a few things to add to the bag if they know you travel a lot, and sometimes they have travel-sized sets on offer to buy or free if you buy 2 products etc). But Muji also do various sized pots as well, just no markings. (Honestly, I don't work for them but their items are very useful). I have a small Ziploc plastic bag (probably had a set of instructions or similar originally) to hold a small wodge of cotton wool pads for cleansing. And a toothbrush in each too.
I have an almost full second set of makeup (as I am not travelling so much right now so am letting things get finished without replacing) - foundation, pressed powder, multi-colour blush, MAC 4 eyeshadow box (that you can change the colours in yourself so allows seasonal or mood changes etc), eyeliner, travel sized mascara (only black - I generally wear black-brown at home so that is a compromise), lipliner and 2 lipsticks. I have a full smaller set of brushes that live in that (shorter handles) and a couple of short emery boards as well.
I keep the smaller sized washkit in a clear plastic zipped bag that I got (M&S, Boots, Muji, somewhere like that) and airport security are generally fine with it. The bigger one in a proper fabric bag as it would be checked luggage, and has things like a tweezers, nail scissors, ear buds, etc and stuff like a face mask ("single serve sachet"), relaxing bath stuff, deep conditioner for hair ("single serve sachet" again) so that I can do a mini-spa/pamper night/morning one day if I have time.
I also keep any sachets that I find in magazines etc - they are handy for trips when space is tight. And generally, hotels have shampoo and shower gel in the bathrooms and can find toothbrushes etc if you forget those, so I don't always worry about having them with me.
But the washbags live in my travel bag, along with a hairbrush, pocket pack of tissues, small ziploc of firstaid (painkillers, spare meds that I usually take, couple of plasters and a savlon wipe, motillium, immodium etc), adaptor for EU, spare cable for my phone, pen and a hotel notepad (the tiny ones), and a packet of sweets/cereal bar (I always have something to eat - just incase of delays etc). In summer, a kids solid stick sunblock is in my washbag too. So I only have to throw in clothes and shoes, and electronics/papers.
The last muji item I will mention is a small polycarb box with dividers inside, that I use for earrings, rings and small necklaces. I have a small zipper pouch for larger bangles and necklaces.
DH has a great travel wallet for his passport, tickets/boarding passes, travel loyalty cards, travel paperwork etc. I cannot find one I like. But I have a smaller wallet I tend to use travelling, when I only need my bank cards and a couple of travel loyalty cards but not the local coffee shops/supermarkets/library card.....that takes up loads of space. (I do keep a book of stamps in both wallets - as you never know when they're needed!).
I do keep a cable for the phone in my bag, and my work bag has a rechargeable battery pack built into it which is handy. DH has a power pack which is separate but finds it invaluable. We both only have a single usb port plug, and need to find multi-point versions (at least some airports now have usb charging points too).
DH tends to wear that days' tshirt and boxers in bed when travelling although pjs at home. I have various M&S jersey versions that actually roll pretty small - and no one sees the wrinkles except me.
If I am going on a few days trip, I sometimes use a couple of packing cubes to organize myself - or if only a 1 day trip and I want to use just a work bag and not have a case. (My workbag has separate compartments, but I have had to open the "personal" one in company in the past, so the cube is handy to save my blushes!). But it really depends on the trip as they also add a small amount to the weight and slightly restrict how you can fit things together.
I don't tend towards umbrellas - annoying, get things wet rolled in the bag, gets me and my hair wet when in use!, and airports don't like them. So I always check the weather forecast ahead of time and bring my raincoat if needed (I commute by train anyway so have an Ilse Jacobsen coat past my knees which is perfectly smart enough for work and travel). In summer, I have a mac-in-a-sac if the forecast is dodgy.
I generally bring a couple of teabags from home and buy a small bottle of milk once through security in the airport (for the hotel), and my bottle of water for drinking en route. But if 3+ days trip, I will get a few more bits in a local corner store/supermarket on arrival, like fruit, a bag of nice crisps, something I can eat in the room like cheese and crackers, and a packet of biscuits. (I've very occasionally been known to get a bottle of wine or something in duty free for long enough trips that I know I will get a chance to unwind in the room and/or work there in the evenings rather than having to go out networking over dinners).
Yes to capsule wardrobe. I favour black trousers and black polo neck jumpers a lot in winter - with a jacket/cardigan over if really cold, or changing statement necklaces. I have about 6 jumpers in black, and another few navy, brown, red etc (M&S ribbed polos were great for a few years).
Otherwise, I try and have the same base colour so use the same pair of shoes - I have a pair of navy heels and a few different outfits to go with those, or black heels and a few dresses, the trousers, and a jumper/A-line skirt option as well. Statement necklaces and scarves can change up an outfit quite well. When weather is a bit warmer, plain black long sleeved tops travel well too (Lands end ones are particularly good, but so are H&M ones, M&S have gone too cheap and see through).
I always have a spare knickers, socks/tights and top/full outfit (depending on what I travel in) and wash gear in my carry on, as I have had luggage lost in transit in the past. Airlines will compensate for what you need to buy, but you need to get as far as shops and sometimes that may be lunchtime on day 2.