Most upcoming "btc layer 2" are not actually L2s.
Do your research, and go after the ones that actually meet the definition of being a BTC L2.
Currently 20% of Bitmaps are staked in a EVM chain, not a BTC L2 🤯
What is a Bitcoin L2? Here are definitions that align with how the rest of the crypto world thinks about L2s:
zkRollups - publish data on L1 to inherit data availability and consensus guarantees, and use validity proofs to guarantee state roots correctness. Not possible on Bitcoin without Bitcoin itself verifying validity proofs (requires covenants + op_zk_verify).
Optimistic Rollups - they publish data on L1 to inherit data availability and consensus guarantees, and use fraud proofs to guarantee state roots correctness. Not possible on Bitcoin without Bitcoin itself being able to handle the fraud proof challenge response setup.
State channels - they don’t publish data onchain, but require users to keep their data in order to exit. They’re not general and require users participation. This is like the Lightning Network. There are scaling challenges here, so not a silver bullet.
Plasma - they don’t publish data onchain and use fraud proofs for disputes. Similarly to state channels, users are required to keep their data to correctly exit. They don’t require users participation, but they do not support general computation. Maybe Taproot Assets or RGB could fall in this direction eventually as they are interesting P2P protocols.
Then, by introducing more trust assumptions in the data availability layer, we can also specify the following categories that fall outside of the L2 boundary:
Validiums - These systems rely on validity proofs and data published externally. Not possible on Bitcoin right now.
Optimiums - historically named Optimistic Chain. Those systems rely on fraud proofs and data published externally. Not possible on Bitcoin right now.
A lesser way to do bridging can be accomplished with BitVM (trust minimized bridging rather than trustless bridging). In this case, we get the following:
BitVM Rollup - These systems use a single prover, multiple verifier model to bridge funds. You trust that 1-of-N verifiers is honest. You still use Bitcoin as a full DA layer. For Bitcoin right now, this is the best that you can do, so it seems ok to include this as a valid L2 category even though you have 1-of-N trust assumptions.
Sovereign Rollup - Not mutually exclusive from the BitVM rollup, a sovereign rollup uses Bitcoin as a full DA layer, and then relies on light clients so users can verify all L2 txns/proofs themselves. This gives you trustless txns on the L2, but you still have trusted bridging.
On Ethereum, it doesn't make sense to talk about BitVM Rollups or Sovereign Rollups because you don't need them. You can build more trustless solutions on Ethereum.
On Bitcoin, it makes a lot of sense to talk about BitVM Rollups and Sovereign Rollups (for now), but we should all readily acknowledge that these are necessary stepping stones to build more trustless L2 solutions in the future (once Bitcoin can handle it).