There is a not-so-minor problem with following up on the events you suggest.  If the Romulans ever figured out that they were duped into fighting the Dominion War, then it would stand to reason that they would have reason to declare war at least on the UFP because of that.  Maybe not necessarily openly, but...the Tal Shiar would not take that knowledge lying down.<br/><br/>There's potentially 3 things I would want to follow up on.<br/><br/>While the original was destroyed, there's nothing to suggest that the Crystalline Entity was the only one of its kind.<br/><br/>If you want Cloak & Dagger, there's always Section 31.  While the STA books take place pre-end of DS9, they present a number of theories about who and what 31 is.<br/><br/>Finally, there's the Maquis.  We don't really know much about what - if anything - of them survived.