I think .ist has a great potential if you're planning on making an investment on new tlds. There are thousands of domain hack possibilities like dent.ist, orthopaed.ist, agnost.ist, chr.ist, jesuschr.ist, activ.ist, rac.ist, antirac.ist, art.ist etc... Almost all medical professions end with ist, besides the hundreds of meaningful dictionary words ending with ist, almost all words become a meaningful and brandable name taking the ist extension. Take application.ist for example, there's no dictionary word "applicationist" but this is definitely a domain with high potential. It magically becomes meaningful.
LLL.ist's also have a great potential because it's a short tld, unlike many other .gibberish new tlds this extension can produce nice and short domain names. Especially LLL.ist's containing keywords like usb.ist, ttf.ist, css.ist, psu.ist etc. have the potential to make money imho. Most LLL's are already gone, you'll have to try hard to find one.
And of course high search volume single or double keyword .ist domains will make money and many of them are still available right now. Let's face the awful truth, you can't find and buy a .com for a reasonable price anymore and if it's about making an investment, you can definitely invest in .ist with highly sought for keywords.