Timeless concludes the story of Alexia and Conall. The storyline involves them being summoned to Egypt and learning more about the actions of her father and the spread of the God-Breaker Plague which first appeared in the second novel.
Overall, I did love the series. It's a fun blend of romance, adventure, supernatural, and steampunk and I look forward to reading the rest of Gail Carriger's works.
After I finished this, I did a quick readthrough of the first book, since I hadn't read it since July of last year. I did love the series, but I think Soulless is the best of the bunch, primarily for the nature of the relationship between Alexia and Conall. Obviously they were together the rest of the books, but there was more of a fun verbal sparring that occured moreso in the first book than the rest.
Gail Carriger did a AMA on reddit a few months ago that I had seen, but not really read since I hadn't finished the series. In one question regarding writing series, she said that she tries to write each novel differently and almost have a different genre/style for each book. I think that's why there is such a gap between the first book and the rest. The first book is very historical/paranormal romance, but the rest of the books are part of one larger storyline, which is still a bit ongoing.
I do look forward to reading her next series, in particular the ones about Prudence out next year :)