Brave man! These kinds of things are so very personal that my first reaction is to tell you just to get a gift card to Knit Picks or Webs and let her go wild. Which would be far easier than reading the novel below.
I have Knitter’s Pride Dreamz as well and love them. Note that they are birch, not bamboo. Plus, Webs is a delight to deal with should things go wrong with them. They’re pretty much the same as the Knit Picks ones, just less expensive. I don’t particularly care for metal needles and work almost exclusively with animal fibers like your wife. (See, personal.) So I can’t speak to any of the metal versions but can vouch for the brand overall.
Three others that come up a lot, especially for those who like metal are Addi Clicks, Chiagoo and Hiya. I do have a couple of fixed in the last two, but can’t say anything special about them. They get the job done. Whee. Hiya makes some stainless steel sharps that come in fixed and interchangeable, but I’m not sure what kind of cable they have. I believe Addi also has some sharps. Definitely do not recommend Boye. I have a set of those and the needles are too long for small projects and the cables super stiff. (Which is a personal thing again.) Actually, if you’re looking for nicer stuff, Boye, Denise, Clover and Susan Bates should not be on your list.
I love, love, love having interchangables, but they aren’t for everyone. As add ons, consider getting a couple of extra cables in different lengths. I also have the connectors so I could connect different cables to get a longer one. (I’m pretty sure I have enough cables to go down my whole block!) Since I mostly knit hats, I have about five 16" cables, then one or two of other lengths. Addi, Knit Picks and Knitter’s Pride all have kind of sample sets that would let you try out just a few sizes.
What kinds of things does she normally knit? Are we talking about yarn to make lace shawls or bulky cowls or just for her to play with? Sweater quantities or just a few balls of different things? Does she have a specific project she wants to make that you can buy the yarn for? (The idea of buying sweater yarn for someone without knowing exactly what they want makes me break out in sweats!)
Colourmart is a good place to get sweater quantities of finer yarns in lighter weights - extra fine merino, cashmere, etc. They come on cones and are good quality. Knit Picks has been adding to its fancier yarns in recent years - lots more alpaca and merino and silk. I have friends who adore the Chroma and City Tweed, while I love the Gloss and Capra. (Note, I mostly knit in workhorse wools, so that would be the Wool of the Andes at KP. Love it as well.) And it’s fairly easy to get to $50 for free shipping there.
Webs is currently having a 30% off on Noro, which is fairly high end. (I can’t stand the coloring, but the yarn is great.) They carry lots of different brands, and I frequently browse through the closeout stuff just to grab new things to try. (I see they have Techno in there again. Awesome yarn.) Other upscale brands with rabid fans that they carry would be Madelinetosh, Dream in Color and Malabrigo. SweetGeorgia, Spud & Cloe, Juniper Moon and Wool and the Gang all have nice merinos as well. Valley Yarns is their house brand, and it’s quite good quality. I have friends who adore the Berkshire bulky. You could build a nice collection of single skeins for her to try out.
And they all lived happily ever after. The End.