Siopis is very good in my opinion. I just saw the highlights of Atromitos vs Panionios, he works for the team, he is also fast and very agile with a good technique something we don't see in our national team's midfield.
I haven't seen much of Kourbelis so I can't comment about him. I agree with Amorgos about needing to have runners in the midfield. Skibbe has played Maniatis and Samaris a lot in the midfield. Maybe it's time to play Siopis alongside either Samaris or Tachtsidis.
Also Tasos Donis has potential to be in the national team for a few years. Retsos does too but our 3 defenders have it covered for the next 5-10 years.