Sharepoint - Alternative options for InfoPath on SharePoint Online

You should be aware of the code-behind on an Infopath form is not be useable in Sharepoint Online. for more details check Can I use Infopath code behind in SharePoint online?

Regarding alternative ,

• You can build your custom form via SharePoint add-in to get start check SharePoint Add-ins
• You can take a look at PowerApps for building and using custom business apps that connect to your data and work across the web and mobile – without the time and expense of custom software development. For more details check SharePoint Designer 2016 and InfoPath 2016 in SharePoint Server 2016, What’s the alternatives ?
• Check also licensed products like Formotus for InfoPath Users or Kwizcom forms.
• Check other alternatives at What are the alternatives to InfoPath

That's a good question and I believe many different folks want to know the right answer.

In reality we have many different options, paid or not, third-party or custom.

I just want to give you some options I'm aware of through the internet.

First of all there is an official opinion from Chris McNulty, Microsoft Senior Product Manager:

We’ve talked about ‘we want you to get off InfoPath’ but we never gave you a good answer before. We’ve really learned from that. And now we have the answer, and the answer is PowerApps and Flow. They are the successor to the InfoPath / SharePoint Designer model.

He gave an interview at Ignite in September 2016. The link.
Other useful links:

• 3 things to know about PowerApps as an InfoPath successor
• 5 Alternatives to InfoPath for SharePoint Forms

As far as I know Infopath forms with code-behind are still possible in 365 (as long as the form goes to a Forms Library, rather than acting as a custom data entry page for a List), but you would be best emailing 365 support directly to confirm.

Sandboxed code solutions have also been removed from 365 so if you need custom code to run you need to use a Sharepoint App along the lines of the new app model MS are currently pushing as the new standard.