I think that layer 1 should include all the financial elements:
account balances, transfers, staking, inflation, voting, rewards (and ideally trading).
This would enable:
(a) A new community (from outside Hive) to launch a simple-and-secure HMT on layer 1 without having to worry about any layer 2 complexities or security concerns.
(b) The Hive reward pool to be moved from HIVE to an HMT (which I think a lot of people still support). This HMT would still be on layer 1.
These "core" layer 1 elements should be free (apart from an initial set-up cost for the company / community setting up the coin). This is one of Hive's most attractive elements compared to other blockchains.
They should also be fast, i.e. 3 seconds (or even less in future). Fast-and-free.
More complex / specialised elements and other smart-contracts can then be on layer 2.
The one area I could see moving off layer 1 is post / comment content. This could be on a side-chain.
This would reduce layer 1 blockchain size substantially / prevent bloat in blocks as Hive becomes more popular / allow improved speed?
Essentially here I think we want the core "coin / financial" layer to be fast and secure. There's potential for a little more "give" when retrieving / storing content.