Easy in the non-coding way and me just sitting on a keyboard coming up with idea’s.
Step #1) Keep the Trade-Up this is getting pretty advanced however with a few tweaks it might work for skins only or keep the perks and we can pop them out!!!
Step #2) Create a store for perks! Just like the Runes in LOL. Come up with a way to buy perks. I would also, maybe in the future like to see stacking perks to gain full strength… 4 perks stacked on a card equal 100% of the overall perk.
Step #3) Go all Diablo 3 RoS on the Cards and treat them like weapons in D3. If I have a founders card and I want to remove the current perks its 100 Credits Each perk. So to Whip a card its 300 Credits. Then add a perk is 100 Credits times 3 perks is another 300 credits. If stacking perks to gain 100% effectiveness makes it into the game then 4 perks per slot time 3 slots thats 1,200 Credits.
Step #4) Expanding step3 is that each card (Skin) could be aligned with the perks you want.
Step #5) Come out with new Skins!!
Step #6) Come out with new Perks!!
Step #7) Come out with Bundles or Perk and Skins
This is still not pay to win because you get credits in game. People can buy the perks to speed up the process and have max perks on all their cards or you can keep swaping perks if you dont want to pay for perks.
More Mercs --> more Skins --> more perks = a lot of combinations and a constant money flow into SD
Extra - What about Guns!?!?! I’m going to hold off on this for now.
Other Things - This might also work for things that arent in the game yet? Swaping and replacing on things if you know what I mean 