I would use TypeScript (I love Javascript but TypeScript definitely fits the job better) or Rust
I would not launch it without a consensus mechanism
I would make Hive the main "utility" token of the sidechain
I would make the whole Hive pegging mechanism trustless (multi sig?)
I would build the account history plugin directly in the main software (no separate tooling)
I would build a validation mechanism that would help detecting invalid transactions before broadcasting them
I would make it token oriented rather than contract oriented (that would make the whole codebase cleaner and more performant)
I would find a way to incentivize people to run nodes (DEX fees? other fees?)
I would build a REST API instead of a JSON RPC API