Дабы не сильно нагружать сервер, сделать настройку синхронизации по времени. Часто встречается с отслеживанием изменений, но это можно оставить на потом.

Желательно делать универсальный доступ. Например чтоб можно было бы подключится к своему SYNOLOGY. Но на первом месте гугл.

need this feature as most of the apps provide google contacts and calendar sync

Currently, the product only provides sync with the built-in DAV server. For performance reasons, we also maintain database cache of the data - for instance, that eliminates the need to parse vCard / iCal files every time user needs to run search. Thus, working directly via DAV protocol isn't an option for now.

However, in the future, we do plan to offer sync with the external DAV servers - and since Google seems to offer CalDAV access, that's going to work for syncing with Google Calendar.

Would be great to get some feedback on this, if it's even considered at all.