Sorry for the long reply, just discovered the proposal. It looks interesting! Why not to combine weak-blocks and transaction blocks then ? I need to re-check the papers but I guess needed modifications to block structure are doable via a soft-fork.
In general, at this point it is better to have non-breaking protocol updates via soft-forks. Which, for observable future, means:
- Sigma 6.0 update to add new instructions and types (and update will be done similarly to 5.0 so with old clients downgrading to SPV security on new blocks)
- sub-blocks (transaction blocks?)
Non-breaking protocol development is needed to avoid breaking projects on top of Ergo, forks, and so community splits, and also to polish Ergo as a kind of digital gold.