Archival nodes also play an important role in the network, however it’s ignored by most blockchains.
My idea is to get archival nodes paid so this will encourage people run one.
IMO as the network grows we should prepare for a large ledger. Each node can store only a small slice of the whole blockchain (e.g. 10,000,000-20,000,000 blocks). The reward should be direct proportional to how many slices one stores and inverse proportional to how many people have stored that slice. Validating can be something like how Filecoin does. RPC functionality can be handled to ‘meta-RPCs’ who stores the corresponding archival nodes to each slice. (Maybe meta-RPCs can also validate and reward archival nodes.)
However this is not a priority yet. It should be worth considering after 1-2y, when 4 shards are all functional and have heavy traffics. I’m just writing down my random thoughts.