I like the feature myself, and I can see it being useful indeed.
About the solution, well, without looking at all possible effects, personally I am not fully happy with this possible implementation, tbh. To enumerate quickly (might not be an exhaustive list): it smells like a clever hack to me (yes I know not everyone will agree with this), it has effects on usability because it assumes equivalence between read and ignored (for example, ignored boards doesn't behave this way, it still shows you topics as "New" when you look at the board index, this would remove the "New" mark from ignored threads; if this is how it seems to me, meaning inconsistent, then, well, inconsistency in UI is usually bad for the user IMHO - I can be wrong on this particular user expectation however), and I think the implementation needs to be a bit more complex in reality than it was done there (to deal with: the user reading the topic by chance, or the user marking as read/unread), meaning it can also be a little more risky than it looks at first sight.
(meaning ... not for 2.0: if features should be banished from 2.0 at this point, even a bit risky features more so

)
But, if the other devs agree with it, I won't be against. That's why I will put it for discussion.