I'll be buying a trail or cross country Mountain bike in the Spring. I'm waiting for the 2019's and the time and weather to do rest rides.
I'm certain that it will be a hard tail with a dropper post, air shocks and hydraulic disk brakes. Most likely it'll have 27.5 or 27.5+ wheels/tires and I'll probably go tubeless. I do basic maintenance on my current bikes, but I'm unfamiliar with disc brakes, shocks, through axles, dropper posts and other features not on my road bikes.
I have some coupons that expire at the end of the year that I can use to buy any special tools I'll need.
Your suggestions would be appreciated.

Torque wrench is handy.
As is some Torx and Allen tool bits.
Air shocks - you'll need a shock pump.
I thorougly prefer the kind with a separate open/close feature.

Tubeless tires "require" a high-flow compressor to seat. There are plenty of workarounds being used by those w/o compressor access.
It can go years between uses, but when it's due, a real bleed kit makes dealing with hydro brakes considerably easier.

A brake bleed kit when you get to that point.

I am lazy, seating tubeless tires and brake bleeds I pay the pros.

Everything else, I would agree with. Good set of metric allen keys, torx, good torque wrech with adaptors if very handy. 15mm wrench. I also carry a hand saw with me (lots of tree fall) multi tool for the ride, good hand pump, floor pump and shock pump. Depending on where you live wet/dry chainlube (maybe both), depending on where you live good cleaning kit with appropriate liquids is key. Micro fibre rags. Bottle of bearing grease.

Not a tool per set, but I also tend to put frame protector on my bikes. 3m makes something that works too. Saves it from basic dings and scratches in key spots.

Thank you all for your responses. Since I have everything but tubeless 'stuff' and an air shock pump, I'll wait till I get the bike as it may come with it. (thank you Kapusta)