About the evolution system!!
In the original games, Digimon evolved into another digimon based on their success rate. Which means, when they had enough experience, they would evolve, but the results depends on how many battles (in percentage) the digimon had won. For instance, Agumon would evolve into Devimon if its success rate was 40% or less (if I'm not wrong), or in Greymon if it had a success rate of 80% or more. Correct me if I'm wrong, but the system was something like that.
Also, note that two digimons can evolve into the same digimon, given the right conditions. For example, either Agumon or DemiDevimon (PicoDevimon) will evolve into Devimon if the right conditions are met for both. Or, Lilamon and Lilymon can evolve into Rosemon - being Rosemon the "right" evolution for both. Of course, there's no set evolutionary path, but there are... "right" evolutions for certain digimon. As it's with Agumon and Greymon. BUT, some Digimon have two or more "right" evolutions. Say, Veemon's right evolution can either be ExVeemon or Veedramon.
DNA (Jogress) evolution: THE RESULTING DIGIMON DEPENDS ON THE ORDER THE DIGIMONS WERE PUT. ExVeemon and Stingmon's evolution is Paildramon, but Stingmon's and ExVeemon's is DinoBeemon. See how things change, if you change the order? However there's a system I liked a lot. In the Wonderswan versions, at the time of jogressing two digimon, the result will be a digitama. Every digimon has their own "jogress points", and they can evolve into adult (champion) if they have two jogress points, and to perfect (ultimate) or ultimate (mega) depending on how many JP they have. Some mega digimon requires 40 JP, just imagine how many jogress would you need! Now, the digitama I told you about has this advantage: it's JP will be the sum of both parent's JP, and will learn both parent's skills.
There, I threw a bunch of ideas, pick the ones you like the most! ;P