I think it is just a matter of "new marketplaces". Paypal benefited from the eBay marketplace ...and found casual online merchants.<p>Generally, marketplaces get hooked with very few mainstream payment methods.<p>Next time there will be a completly new marketplace, with new types of merchants and new market mechanic...you will see a new, major player...but it has to be disruptive to PayPal.<p>Online to Offline is a new marketplace in many ways...looking for a unified payment. If i am not mistaken, the next big payment player may come from that....
Anti fraud detection.<p>It's an issue when you reach scale, hard to detect, and if you end up being ultra sensitive you end up behaving like jerks freezing accounts for no reason (ie what PayPal does today).
You just have to be proactive with paypal..contact them before launching, stay in touch with them to resolve/ prevent any issues (hopefully) before they arise.