There's new dk skill styles, they are sold by stylemaster npc (there's one in Belkarth near the outfit station).
But I just waste my gold on collecting crafting recipes. Housing in general is a constant gold drain.

It depends. First, you can invest in a steady income-alts, to earn even more gold (buy leveling service for alts, buying materials and speed-ups for leveling and crafting, purchasing motifs and recipes for them, etc.).

Second, if you’re into trying out different classes, builds, etc., upgrading your gear sets can get pretty expensive, so buying materials to upgrade sets for new builds is also an option.

Third, if you’re a “housing guy,” then furnishings, plans, houses, etc., etc., etc.—this is basically something where you can never have enough gold...

Or anything from the Crown Store—while you can’t buy anything from the Crown Store directly with gold in-game, people who can send gifts sell anything from the Crown Store for gold this way (there’s even a Discord server that specializes in this trades), so this is also an option.

That’s pretty much what comes to mind off the top of my head regarding where you can spend gold.

For example, I’m at a stage where I don’t really have much to spend my gold on (I have one class that I enjoy and I’m not interested in trying others; I’m not a “housing guy” ...), so I decided to start collecting Outfit Styles (including those from Crown Crates, Arms Packs, etc.). So, I first created a second account and invested in additional characters to increase my income. Right now, I make ~4 million gold a week, and I’m slowly starting to buying Arms Packs, Crown Crates, and the like from players who can send gifts, so now I have something to spend most of my gold on, and I know it’s a long-term run, but still, it’s fun for me.

So find what you enjoy, and that’s where you can invest also your gold.