Missing functionality: iCal feeds for groups. It would be good to have an iCal feed for all subscribed groups at the minimum.
Having the possibility to add groups to the feed even when not subscribed to them could be useful but may be too complex (for when you’re interested by a group but have not decided yet to join it - so basically at that stage all you want is notifications of events and you may decide to subscribe to the group or stop notifications after having been to one event). If you decide to implement the latter, then it would be useful to have super-selections i.e. for all the London groups (same as Meetup).
If the aim is to eventually switch off Meetup, this feature would need to be in place beforehand; it is an essential feature (currently of Meetup) for me and likely for many others too.