Native Custom Fields in joomla is an article feature, i dont know how ES, EB can use it. As they both are totally custom and do not rely of Joomla articles.

Now with custom fields its possible to make a blog system with Native articles and maybe achieve 60-70% of what EB does. But i think EB is much more specialized.

The best way for EB would be to evolve to use native Joomla articles as an underlying blog engine and have its layer of features on top, this way EB can become much more leaner, slimmer and meaner! (Latest EB has already been on a diet course and is looking slim)

Note: I would view ES support as #2 priority behind EB

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Update:
1) The feature should be available to Joomla users via ACL selection (i.e. Joomla admin specifies which user group can access this feature).
2) The feature should pay attention to the available Joomla User custom fields such that if the front-end article creator selects Joomla User "Mark Lee", then any selection from any custom field you guys create should be 'filtered' to reflect associated content for that Joomla user.
3) The UX should be simple and not require plugin type syntax understanding (even though it's relatively "easy", I'm looking to support users entirely unfamiliar with the concept).
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I would like to see stack ideas support Joomla article custom fields as a way for front-end article creators to create a page that includes content from their components. So, for example, a user could create a "landing page" that serves a purpose greater than just the ES user profile.

Stack Ideas would create Joomla Article custom variables that allowed insertion into Joomla artlces, by a Joomla end-user with article creation rights, to select an EB post to include in the article, to select an ES status post, Group, Event, Gallery or an ED discussion or Komento comments to be included in the article page. In this way we enrich the value to the site community members at the same time of the site admin's who seek to add value to the site.

In a way, I'd suggest the "Joomla-article-custom-variable" is to the Joomla Site Member as the "Joomla-Module" is to the Joomla Administrator.

I have a particular need vis-a-vis an offering I am bringing to the community for: 1) a mobile apps solution to build for the Joomla site admin, or to empower developers to create and publish themselves with our platform, mobile apps for Joomla sites and 2) supporting native mobile messaging for Joomla sites (stay tuned, will be posting related info. soon mobilitywave.com).

One user scenario, for example, is to support the QR scanner in our mobile app platform. Site members / vendors can create QR codes that can be scanned and resolve to their "landing page" I referred to above (FYI a QR code is simply a code for storing URLs or other information for reading by the camera on a smartphone, and it can contain other useful information in a query string enabling you to pass field information as well).

Now, this simple user scenario could simply be the ES user profile, but you can see maybe a greater ability to craft other uses of these "landing pages" outside of the purpose to share information about the social network member (use your own creative faculties

FYI to Stack Ideas developers: I view a custom field to insert an EB post as the #1 priority, if you guys think it's something to include on your product roadmap. Why not another existing mechanism? For ease of use by non-developers, to expose a simple modal window from where they pick a post.