cessna, grob tutor, tweety bird, talon, I'd honestly love any (or all!) of these within DCS.
Firstly because, as mentioned above, they would be great tools for actually learning airmanship and navigation, open up a lot of opportunities like pilot training campaigns, virtual training squads and servers, what have you.
Secondly because if they draw players to DCS that would not otherwise have come, great! Also, if they're easier to make and provide an alternate revenue stream for the developers that bring us combat planes, super!
Lastly, one of the things that really make DCS great for me is the sandbox aspect. In addition to being a great combat simulation, it's at its core a great flight simulator. I can fly planes from several eras, over beautiful landscapes, doing basically whatever I want to be doing. I can fly realistic(ish) combat operations, I can fly over the top comedy scenarios, I can fly competitively, cooperatively or alone, and sometimes I just want to get in whichever plane, enjoy the flight dynamics and take in the sights.
I say the more variety in planes, the better! Some argue that there's FSX and Xplane for that, but I'd rather my money go to support DCS developers