Klaviyo forms only allow subscribing submissions to one list. That has a whole range of problems: 

• Subscribed to list trigger only works the first time someone subscribes, so running specific campaigns with perks existing subscribers also need access to, is not possible. 
• If added to a new and unique list, you will have to manually merge lists if you’re keeping a “main list” for all subscribers. 
• If you use Klaviyo forms to send gated content, you will have to make unique lists or risk existing subscribers not receiving.
• You can’t use Klaviyo forms for signing people up to ongoing events, since you would have to build a maze of workarounds to store event data in attributes and trigger emails from segments. 

And many more like these. Everything could be fixed by simply allowing Klaviyo forms to trigger an event when submitted. Just like every other third-party form tool can or already does. 

• Zapier allow all third-party form tools like Typeform to trigger events when submitted
• The Facebook integration does this by default with the event “Filled out Lead Ad” 

This would open up a broad range of applications and use-cases for the forms that are currently very limited to handling first-time subscribers. 

Please upvote if you desire a solution for this, or comment with your workarounds if you have any good ones