Is it do-able? Yes, certainly.
Is it complicated? Also yes.
Landlord or member of their family wants to live in the property.
As the potential for abuse of these grounds is clear, any such new grounds (and their evidential requirements) would need to be carefully considered.
Simply ending s.21 is not effective if it can easily be bypassed...
(Oh and housing associations will have to come up with a different way of doing 'starter' tenancies.)