After giving it a try, I don’t think it’s a super viable path to change the type. It’s a lot of change that breaks easy code portability, it compiles without problems while certainly bugged. flags_t could be ScriptFlags, or whatever for better readability.
edit: I did experiment with using typedef std::bitset<64> flags_t; for script flags, it could be a good solution long term, but the imminent pain is even worse. Also doing so noticed that the flags as unsigned int are indeed exposed on APIs. and honestly I have no idea what to do with that.
edit: Context for picking up this thread again: LNhance + C3PO has enough script flags left, but I became increasingly convinced that C4 should also be part of the conversation, because activating CAT without CCV carries the likelihood of greater annoyance with new opcodes as opposed to the immediate desire to soft-fork in further optimizations.