no problem :-)
As far as I can see, there's following things:
Should be no problem: Add a new field, store in the db, make it visible on template only if ADMIN
This sounds like there has to be some kind of logic behind this. "I'm interested in further informations, please get in touch" sound like the admin(s) should be informed of such things. Otherwise you'd have to check manually... Not so easy
What do you mean? At the moment you can choose to have all entries "disabled" and not showing on the guestbook until "enabled" - except the user is a registered and logged-in member. Comments on the Entries are only possible for logged in users to write.
I'm not sure if the guestbook is the right thing to use as a discussion board. That's normally done with a forum where you can even have multiple threads etc. Doing this with the GB would mean a lot of modification away from the basic purpose...
One thing you made me aware of is this: Maybe I'll implement a possibility to add like 4 or 5 additional form fields which could be enabled and labelled as desired to catch some more user data and be shown on the GB. I'd have to think about maybe having this a bit more dynamic like in the userforms module where you can have different types of fields, boxes, radiobuttons etc. as a form. Could be interesting, especially when it needs to be multilingual. But adding further validation or actions away from the basic things like isNumeric, notEmpty etc. should be done as a customization and not within the GB development.
By the way:
Are there any feature requests (general question)? :-)
Because at the moment I think I'm done with 2.4rc1 compatibility issues - found no more bugs so far and since SilverStripe announce there will be no more API changes from this development stage on... (for 2.4, of course) ... I'm ready for some more developing ;-)