There is no officially published list of new functionality. For now we can only speculate.
One thing is certain the ‘new’ API would be the ONLY API supported in Odoo 9.
Backward compatibility will be removed in Odoo 9 and all future modules must be developed using the ‘new’ API.

Update from #OdooExperience is that the “old API” will be supported until v10 or v11.

Fabien Pinckaers latest comments about Odoo 9 are that there is no deadline. Odoo will release new version when it’s ready. Most probably in 2015. You can read full comment here.

Friday, May 8, 2015 – Update about release date by Alexandre Vandermeersch on behalf of Odoo s.a. management team:

We expect the Odoo 9 release (both editions) to happen around August-September.

I expect a lot more information after Odoo Experience 2015.

Odoo experience 2015 highlights:

1.) New accounting
2.) Shops and restaurants management
3.) New marketing apps

Also Fabien Pinckaers and odoo twitter feed listed some new features:

– Accounting Module re-write
– Voice over IP Module

– New drag and drop Gantt and smart capacity planning

– An improved forum with social network integration
– UX improvements
– New functionalities & usability improvements in new summernote website editor.

– New call-to-actions for landing pages
– Project management updates

– Call-to-actions for other pages (eg. a bounceexchange clone…)
– A much better mass mailing app (new email designer, more stats)
– A tracking system from visitors to invoice (even before visitors become leads)
– A rule based scoring system for leads (inspired by kissmetrics)
– An open source slideshare clone (share presentations, videos, docs)
– Electronic Signature for contracts, employment contracts, nda…
– Mask designer

– A versioning and A\B testing tool for CMS
Odoo 9 lets you create different versions of your website, save all modifications in various versions, retrieve them or delete the versions. Now you’ll never lose any changes you make to your website. Odoo 9 allows you to test different versions of your website and to choose the best one by simply using A/B testing.

– Gallery snippet for the website builder

– A new building block for eCommerce  – now you can describe your product details in a table.

– Better SEO with promote adapted to languages
– A link shortener and tracker (bit.ly integrated with UTM and stats)

Update:

Watch the keynote from the Odoo experience 2015