.de seems like a solid investment depending on the keyword. IDNs in .de have also taken off and have been selling for xx,xxx... I wish I knew German to buy some of the IDN/ASCII domains... but I've already screwed up with other languages that I don't wanna take the risk.
Not sure about restrictions, but I think it is a good idea.
Some German IDN.de Sales that just took place, plus there should be more at DNJournal with the .de extension.
ärzte.de - $77,000
reisebüro.de & reise-büro.de $70,000
Just remember if you find a keyword with an accent at the top, it can still be purchased!