Since composer actions are basically just mini apps, why not let cast actions open sandboxed webviews as well? 

Opens up lots of possibilities that are not possible in frames

𝓵𝓲𝓿𝓲𝓷𝓰 𝓽𝓱𝓮 𝓮𝓷𝓭𝓵𝓮𝓼𝓼 𝓼𝓾𝓶𝓶𝓮𝓻 • building @buoy 🛟 • pfista.com

building products /neynar, hacking /farhack, yapping /dylan | dylansteck.com

Agreed, and it's easier to iterate in an iframe with traditional web tech rather than making everything an image. 

Puts more of a burden on the WC team re: security / app review, but arguably a better in-app experience with more functionality. Could help user growth

I have a feeling the majority of embeds in Warpcast/FC clients will move closer to iframes than current frames/meta tags — just so much more surface room for devs & users

100% agree, have asked the same question and would make a lot of sense imo

I am proposing the same for frames as well. Which can create a new set of apps in Warpcast.