Topic tags imported (synced) from post tags
Also a wpForo option to restrict forum tags to existent portal tags. Or the opposite. Or to always synchronise them.
So many things have to be taken into consideration by Cross-posting, which is a marvel and a game-changer for the whole WP platform.
(other demands; synchronise the user if changed after cross-posting, synchronize the date of posting if changed thereafter, synchronize the smileys with wpDiscuz, synchronize the Like/Dislike with wpDiscuz, the private reply and so on)