Though the use of a current IP which involves adventurous characters, used in a series of adventures around World Showcase at least seems like one of Disney's better IP-shoehorning attempts, I'd be heartbroken if Agent P leaves.
Phineas and Ferb and the battle between Agent P and Doofenshmirtz totally works for World Showcase. They came up with a good story, and integrated the challenges in each country very well. I understand Ducktales is current, and P&F has been off the air for a few years, but from what I've watched in the new Ducktales, it doesn't hold a candle to P&F in terms of writing, humor, and originality. Also, just because something is off the air, doesn't mean it's not classic (cough Twilight Zone cough). It's clear Disney views TV show IP's impact and longevity differently than movies.
I grew up with the original Ducktales which I loved and maybe that's why I'm not too into the new one (insert "old man yells at cloud" pic). Same goes with TMNT. But I was a parent during P&F's run on TV and was as into it as my child due to the jokes they'd insert for adults, or Doof's one-liners that completely catch you off guard. It was so well done and I don't get that feeling about the new Ducktales. It could work in WS... but it also may be off the air in 2 years and we're talking about what new IP will be replacing Ducktales.