I'm largely there with you... Cloudflare Durable Objects feel like the best means to implement a sequential number generator. I think a lot about mail and message systems... part of why I'd been really excited to see what CF had started with in terms of mail systems support. They've got really great natural privatives for a first class, modern mail and message systems. In practice, handling updates to threads or simpler value assignment/alias DO is just such a great fit.

For reference, part of where the thought exercises are for is in terms of replicating what might have been a traditional BBS message net, but a massive hub for such activity on Cloudflare hosting. Some pieces burrowed from FTN (Fidonet technology) but some rethinking in terms of more modern capabilities in practice.