As I see the card-game, and how it tends to go, I´d say, there are lots of players (new ones, but also old ones), which are simply amassing and junking cards/creating even more and more "farms" and "storage nations" without any other meanings than to build up wealth or "hoarding", which is to regret, because imo this leads to a downfall of the game over time.
One of the reasons for this kind of behaviour might be, that some of these players would eventually like to start/continue a collection (other than "as valuable as possible" or "my own nation/region"), if they simply could think of a theme/object for it, which offers the option of being "remarkable" and is not already occupied by some other player/s, that is/are collecting those cards for a long time already; it´s relatively unsatisfying to compete with an existing collection of hundreds or even thousands of cards without any chance of ever being "noticed", mentioned and listed in the "collection collection".
Therefore I gave a few thoughts to find some "unoccupied fields", if someone would be in need of hints, which collection to start.
Easiest (and, as I guess, most common one) is to start any "geographic collection", like "your homecountry", if it doesn´t already exist on the list.
Next, there have been a few collections (in season one mostly), that have been started and cancelled over the years, like "Phoenixes", "Real Existing Ancient Nations and Empires", "Unicorns", aso.
Then, there are lots of opportunities to start some specified "volume collection", like the numerous "raider - defender" and similar ones (the "moths", the "tic-tac-toe pictured" might be a good example), or the "two letter nations"; they usually count in thousands, and most of them are rather easy to collect, as they tend to amass in one or a few regions.
Of course, much better would be to find a theme for yourself.
And least, I would like to invent other players, which dont want to or dont have the time, the space,the bank (like me, single nation without farms, three "listed" collections and paid space) or the will and endurance to collect for themselves to participate and post their ideas for collections they wont or cant do for themselves but would like to see in this thread, so other players (who cant decide, what to collect might start such collections); which, imo, would also be an enrichement and boost to the "free market" and safe a lot of otherwise "uninteresting" cards from being junked without even being offered on the market.