RISC-V is the starting-point: a simple Turing complete register machine that is open-source and widely adopted. Any contract that can be written can be written with the RISC-V ISA. The RISC-V ISA allows for "custom instructions." We will find that many contracts are using the same computations (e.g., elliptic curve point multiplication) and so pages of RISC-V assembler can be replaced with a single new custom instruction. And that instruction can be carried out with specialized circuits inside the RISC-V machine (optimized C-code for Ethereum or a FPGA/ASIC circuit for Nexa). I learned about this idea from @GAndrewStone at the "Instant Transactions Workshop" at Lake Garda in 2018. Andrew convinced me that it doesn't really matter how "complex" the scripts are initially because scripts that are proven to be useful can be extracted into new "custom instructions" which can be optimized in software or hardware.