I have 3 website running on Elgg. 2 of my website are based on core and 3rd party plugin but my 3rd (the latest one) website is 100% customised with no core or 3rd party plugin. I have developed all the plugin based on my requirement and using it. You don't have to modify the core. Elgg functionality can be modified via custom plugin and you can almost achieve everything by using Elgg as a CMS or as a framework.
@Virtu Just see what you can do without modifying the Elgg core.
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