Foreign replacement notes are really hot these days. The problem is figuring out what are replacements and what are not. In Canada, for example we have had asterisks, X as the third letter in the prefix and now random inserts to designate replacements. Other countries have variations as well depending upon the issue. Replacements may variously be designated by asterisks, stars, crosses, Z, ZZ, ZZZ, X, XX, Z/1, Z/2 etc, 9/99 and dozens more. Very little catalogue information is available on replacements.
Topical collecting is another interesting way to collect foreign notes. If you choose animals or flowers you will end up with a huge collection. In the last couple of issues of the IBNS newsletter there have been articles on topical collecting. One fellow collects only notes with dictators on them. Another collects notes with Nobel Peace Prize winners. (Sadly, there has never been a Canadian banknote with a nobel peace prize winner..... I therefore vote that we get a new $200 bill with Lester Pearson on it so that the Nobel collection will have a canadian note!!!)
Finally regional collecting is also popular. Try Faeroe Islands and Iceland, and you will have a smaller collection of beautiful notes.