Thanks for starting this timely and interesting topic,
@Internet.Domains, and thanks to all who have responded.
I was actually working on a lengthy two part series on exactly this topic to be published at NameTalent in early January (I will post the link in this thread when it is published). Still developed my ideas, and look forward to learning from others in this thread.
Here are a few of my early thoughts. I hope I am wrong, but the considerable negative trends and uncertainty in financial markets will be a drag on domain spending, particularly at the upper 10% of the market.
I fear that 2019 the sales volume will be down in domain names, perhaps significantly.
I am not sure that .io has not already peaked and don't see a lot of new strength in other generic country codes. As always .com will stay dominant. I actually feel that .org may have a very solid year, both in terms of continued expansion in use and in sales.
The ngTLD space is so hard to predict. I see some hopeful signs in .online, and .top continues to be strong but only in one region and mainly registry. There is not much sales data to support the idea, but I see .xyz as having a better year next year, sales wise. I think for most ngTLD investors it will be a fairly lean year, with big sales now and then but not often. Again, I hope I am wrong.
What niches?
AI both as a topic and in consumer devices will continue hot (and maybe .ai as a generic country code too).
I think ebikes may have a huge year, and see some sales at all price levels. It sometimes slips under radar, but I think the movement to
healthy lifestyle, fitness, happiness, mindfulness will have another good year.
I don't invest in or follow gambling, marijuana or political domain names, so my not saying anything about them should not be interpreted as anything other than that. Clearly a lot of money was made in at least the first two of those in 2018.
Still thinking about others, and doing some research re second half 2018 trends.
Bob
Disclosure: my domain portfolio is worth so little that there is not significant conflict of interest in anything I write 
.
But in case you want to know, I have a few .org, a number of happiness/mental health/fitness names in various extensions, and a couple of lowish value ebike names. 
