I'm the author of TicketChecklist
For all *my* free addons, OPAR should be the primary source!https://opar.perl-services.de/dist/TicketChecklist
If you have any questions about my addons, please do not hesitate to ask!
I will set up a demo system in a few minutes (credentials will be sent via private message).
After installation, you'll get another item in the ticket menu in ticket zoom. There you can add todo items. When you save the todo items, a new widget is shown in the ticket zoom. There you can change the state of the todo items.
You can still edit the todo items and delete/add items via the "Checklist" dialog.
There's also a commercial version of TicketChecklist that has loads more features, but for basic checklists the free version should be enough.