These are definitely long-term projects, just owing to costs, but here's my list: 1. Liberty Seated Quarter Dollars 2. Liberty Seated Half Dollars 3. Switzerland Large Size 5 Franc 1851-1931 4. German Weimar Commemoratives
I can't speak to the European series but the US LS Quarters and Half Dollars is a pretty challenging set by date and MM. And if you're going to be collecting in a high grade, it's even more of a challenge, especially monetarily. Of those two series, I would say that the LS Quarters has the most "space" for a serious collector with academic interests. It's far less researched and documented than the LS half dollar series. I've always been intrigued by the US Capped Bust Quarter series but it's also pretty daunting. I'm well down the rabbit hole of the Capped Bust Half Dime Die Variety collection so I'm not willing to open another gaping hole in my interest and budget.
Any of the Seated Liberty series are a challenge. I need one more Barber dime for my circulated set but have little hope of completing the Seated Liberty dime set I started. Long term I might consider 20th century proof sets.
I have toyed with the idea of assembling a complete set of Danial Boone Bicentennial (US Classic Commemoratives) series (1934-1938). I've always thought Boones' contributions to US history are most notable and the Shawnee Indian nation quite intriguing indeed.
I'm trying to get a complete collection of US reverse proofs (minus the gold coins). I have limited funds so it is going to be a challenge. Also thinking about a collection of 'W' mint coins.