I think it goes without saying that robusta will add a little bit extra to a blend but remember not to go too high a % without trying it as you go.
Aged beans like an Indonesian Florez or even Indian monsoon Mallabah will always add something completely different.
A good quality natural processed coffee will add crema and some character to a blend.
I have found that if im using beans that have similar qualities once roasted into second crack that it might need some high notes(fruity, zesty zing) an ill add a lightly roasted African of some sort..usually a good quality Kenyan.
Its all still trial and error for me so im keeping records of what works and what doesnt...but usually the ones that dont work so well are easily adjusted by different roast profiles of the ingredients or the addition or subtraction of 1 bean.
All i can say is...its all fun and most importantly..its all fresh coffee :)