Premise: It is not sustainable that billions of users send millions of transaction through the block chain every second. It will just grow too much.
if wrong, goto end and post reply.
if true, continue:
So we need to put for example 10% of users into cluster A, 10% into cluster B, .... 10% into cluster J.
Now all millions of transactions are collected for a while (10min?) and all transactions within clusters are settled in the cluster internally. Only if there are settlements to be done between clusters, those go into the blockchain. The blockchain now only contains few giant sum transactions with giant fees that go from cluster A to B, or C to G, J to D, etc. that represent these inter-cluster-settlements.
Congratulations you have just eroded the bitcoin system and established the old banking system again. Cluster = bank. Inter-cluster settlements = inter-bank settlements (aka Target2)

What to do?
Maybe some kind of dynamic fractal clustering, where each cluster, subcluster, subsubcluster,... has it's own blockchain and every time something can not be resolved within the subsubsubsubcluster blockchain it gets transfered to a higher cluster blockchain, bitcoins becomes thousands of blockchains and you can 'subscribe' to the subsubsubsubcluster you like, or something. The blockchain we know now, becomes the inter-continental blockchain, that only will contain transaction/settlements between continents, or Planets, or Solar Systems.