To add to my colleague's excellent answer, I'd add that there are 5 main niches in this game : small - medium turboprop - medium jet - large - very large. Due to the commonality rules, it's better to stick to 2 of those niches, and have only one fleet type per niche(plus one for the transitions).
TXL is not a very big airport. IMHO, it's not possible to play the large-very large combination that works only on the very big airports. So you have the choice between looking for another base for your MD80s after filling 90-demand routes, or spamming lower-demand routes with smaller regional aircraft. Both solutions have their issues. Keeping large only makes you vulnerable to medium-spammers on everything up to 800/1000NM of distance. Adding your second niche right now while you have only 12 large aircraft will increase your commonality cost rather quicker, which is why paddk989 advises against it.
As it's beginner's game and you are based in the European Union, there are probably plenty of airports where you can grow. In a real game world, though, I'd advise you to "lock" your headquarter to deter anyone to attack you there. In current GW2, I'm playing in Poland, and all routes below 900NM and over 20 demand are covered by at least one F28 flight(in 55 seats configuration). In each of the 6 decent airports of the country. That's a bit extreme, but whoever would try to invite itself there would just lose money.