I worked in the business for a number of years and still have friends/colleagues that are there.
From an engineering perspective slot machine design isn't very hard at all. It's pretty much video game design. There's a relatively recent CPU putting down images, playing back video, doing some light/sound choreography, and maybe doing some mechanical motion on some games. Let's put it as slightly more complex than Candy Crush but way less complex than writing Halo. There's also a mechanical component with artists and designers making cabinets, art glass, and wacky add-ons that sit on top or around the machine.
The hard work in slot machine design is in the math of the slot machine. You're trying to design a game that balances something interesting on-screen with enough volatility that keeps players hooked. If you decimate their bank in the first 100 spins that player will never come back. One typical way to manipulate this is through the secondary games or bonus games that pop up on certain reel combinations. At first this was a way to work around certain patents (the most famous being IGT's Telnaes patent) and now players just come to expect various levels of extra action when certain slot reels are hit. Every player likes something different, of course. IGT's Double Diamonds reel-spinning slot is over 25 years old and players still look for it.
The math also has to jive with the gaming board. Certain states will mandate minimum payout percentages, or a mixture of these that average a certain payout. (This is why dollar slots in Vegas pay way better than penny slots. As long as all the action in the casino averages to the legal minimum, they're okay). Your typical slot manufacturer has a number of math/stat MS or PhDs working out the numbers on paper as well as a testing department that tries to verify the math empirically. I'll say "tries" because some designs are so complex and non-deterministic or dependent on player choice that it can't be verified entirely on paper anymore. Almost all American jurisdictions employ independent gaming labs that inspect the code and verify the math as well before the slot is approved to release in a given state. If you can get approved in Nevada, you're usually good everywhere as a rule of thumb.
And then yes, the theme of the slot has also become incredibly important. Licensing pop culture items (movies, TV shows, dead musicians, celebrities, etc etc etc) is a constant fight between the manufacturers to capture and leverage into a usable slot idea. Some think it is creatively lazy to grab the next hot movie and turn it into a slot. Some think that's what you need to do to attract people's attention in a visually crammed environment. C'est la vie.
A slot machine designer worth his salt can mix and meld all of these things into an interesting game. New and different game mechanics are hard to come by, though. When something does hit, you'll see that model replicated dozens of times with slightly different math or new themes slapped on top of it. You throw a lot of shit at the wall to see what sticks.
Business wise, up until a few decades ago, slots were sold by manufacturers and entirely owned by the casinos. In the last 20 years or so this has transitioned into a participation relationship where the manufacturers own, maintain, and operate the games themselves and share a percentage of the handle with the casino. This has made the business a lot more cutthroat because a streak of lousy games can get you kicked off a property (or corporation) entirely. This is the downside of throwing shit at the wall, so you test and test continuously at smaller properties. Participation also allows interesting game mechanics like multi-casino slots (like Megabucks) where the manufacturer is holding the jackpot in an interest-bearing account in their reserves. Think about that when the jackpot is in 9 figures and nobody has hit it for a year.
Anyway, if you're really interested, the big companies are always hiring. These days it's IGT in Reno, NV and WMS in Chicago, IL. There are smaller companies here and there (Konami, Aristocrat, Mikohn) and then a constellation of independent design groups that consult and design games on spec for the big dogs. Most designers work their way up from programming games or designing art for slots but if you have any video game or casual game experience there's always room for new talent.