Yes, the S-15 is one of the next for sure... I have its blueprints drawn and most details are drawn too. I just have to finish some I have now like the F-22 and the Yak-15 to create some space for new planes...
Well, if you want to make planes I can help. But I warn you it's a very time and brain consuming hobby. And it's also a pain to learn 3D software, especially if you're doing it on your own like me when I started with gmax... At the time there were almost no tutorials online...
Btw if you want to start you can begin with this simple tutorial. It doesn't use exactly my type of modeling but it teaches a lot of neat tricks:
http://www.flightsimonline.com/C162/