Currently only seven classification systems are supported, even though this may cover a "majority" of Classification Systems around the world, at least six classification systems are missing out and some are important to several cultures strict guidelines and customs,

Office of Film and Literature Classification (OFLC - New Zealand)
(https://www.classificationoffice.govt.nz/)

The Film & Publications Board (FPB - South Africa)
(https://www.fpb.org.za/)

Indonesian Game Rating System (IGRS - Indonesia)
(https://igrs.id/)

Game Software Rating Regulations (GSRR - Taiwan)
(https://www.gamerating.org.tw/)

General Authority for Visual and Audio Media (GCAM - Saudi Arabia)
(https://www.gcam.gov.sa/pages/32)

Entertainment Software Rating Association (ESRA - Iran)
(https://esra.ircg.ir/)

As listed above, two of these classification systems are used by countries that enforce or utilise parts of Shariah Law (Islamic Law), countries that include but are not limited to, Ghana, Egypt, Chad, Nigeria, Algeria, Libya, Sudan, Ethiopia, Iran, Iraq, Afghanistan, Turkmenistan, Azerbaijan, Georgia (Eurasia), Saudi Arabia, Yemen, Oman, United Arab Emirates and many more Middle-Eastern, Asian and African countries. I believe these should be added to help Islamic families and other families influenced by Shariah Law in their own countries and other countries that don't have Islamic law so they can uphold family values and their beliefs, both GCAM and ESRA have article/title search databases which will aid in finding classification ratings for popular video-games.

The other classification systems listed above are common and some relatively new classification systems, the OFLC are very well known within Australia and New Zealand as the Classification System for New Zealand and has been around for almost two decades, GSRR is widely known as the Classification System of Taiwan, however due to its political nature I'm not sure how easy it would be to introduce as the others, IGRS is a much more recent addition to Indonesia which means it has newer titles but not many older titles and most likely none from pre-2010 and the FPB in South Africa is also somewhat recent but has classifications for modern video games.

I believe these six classification systems will help aid the IGDB Website and add more depth and inclusivity between countries, communities and beliefs, not to mention there isn't much difference between having these classification systems than having CERO, GRAC and CLASSIND.