Given the end of life of InfoPath - supported till the end of 2023, what is the recommended alternative going forward for new development?
The roadmap going forward is via 1) PowerApps and 2) Microsoft Flow. These products are not direct replacements for InfoPath offering a much wider canvas. It's likely, imho, that Microsoft will craft a forms product from the PowerApps and Flow platforms.
I'm guessing more to be said at Ignite. Keep an eye on the Office 365 Roadmap too. 3) Nintex is a strategic Microsoft partner is this area so may still be the right choice and InfoPath still has legacy support for some time but I agree not for new projects. BUT Nintex pricing is insane.
You could also take a look at 4) Lightning Tools or 5) Skybow products, or 6) StratusForms (The commercial version of Forms 7 which was a community solution, but is free for non commercial use.)
https://lightningtools.com/products/lightning-forms/
https://www.skybow.com/
http://www.stratusforms.com/
7) SPARK is web-based forms builders that lets you quickly and easily create multi-lingual electronic forms within SharePoint environment and Office 365.
SPARK which refers to "SharePoint Advance Redesign Kit" allows users, designer and SharePoint admins to easily build highly professional forms in SharePoint platform simply by placing a rich set of drag/drop controls in a user-friendly editor which let them also controlling and configuring the control’s properties, style, validations, rules and formats and publishing these forms to SharePoint lists and libraries by just a click.
Using SPARK, you can easily and quickly create and design controls in your form by dragging and dropping them in the design canvas. You can create simple forms and apply simple business rules without effort, also you create complex and logic-driven forms using OOTB SPARK functions, JavaScript, jQuery and Cascading Style Sheets (CSS). Using SPARK you decide the information your business needs and use a SPARK's controls, tools, bult-in functions, rules and features to design and update forms to your exact specifications.
http://www.itlaq.com
https://youtu.be/UGnOHrNsZqI
8) http://www.flowforma.com/
