Not ECDSA. Not Schnorr. Meet DahLIAS.

Jonas Nick and Tim Ruffing of Blockstream Research and Yannick Seurin of Ledger published a paper presenting DahLIAS, which the article describes as the first formal, secure construction of a full constant-size aggregate signature scheme that works on Bitcoin's secp256k1 curve. DahLIAS combines multiple signatures across different inputs, each from a possibly different signer, into a single 64-byte signature whose size stays constant regardless of the number of signers or inputs. The article says it adds no significant new assumptions beyond those Bitcoin already relies on, and that its verification is up to twice as fast as checking signatures individually. DahLIAS is not a multisig protocol and is incompatible with current consensus rules, so adopting it would require a BIP and a consensus change, which the paper does not propose.