I thought the Xindi arc was OK, but they missed out on a couple of things. One, there was no need to have an attack on Earth - that is the type of thing that would have been referenced in the previous 20+ years of Trek episodes more than likely! It could easily have been a colony that was lost. Of course, that goes back to Braga's famous line from ST:Nemesis - "We've figured out what Trek fans want. They want to see Earth in danger.' LOL.
Two, I though they really could have had an interesting parallel with the Xindi. Instead of multiple different species that intelligently involved on the same planet, they should have made them an actual federation. The juxtaposition between the fighting a federation of allied alien worlds (all being manipulated by the Sphere Builders) and the fractious nature of the planets that would later be part of the UFP would have made for some interesting social commentary.
Finally, they should have made the Xindi disappear at the end. That could have been part of the larger Temporal Cold War arc and given it real oomph as an ongoing basis. At the very least make it understood why a major world nearby had little or no presence in the ongoing Trek saga past this time period.
Continuity is maintained, you got some actual social commentary, and you make the Temporal Cold War relevant in new ways.
As far as the Romulan War went, you can't have it sublight, it's just too dumb. But I think they showed with the holographic drone ship that you could have quite a bit of interesting new developments in the Romulan War.
You could also have made an episode where the Romulans were revealed - however the Vulcans, fearing the backlash, used telepathic means to remove the information from one or more of the crews minds. Of course, if you did that, it would have to be a branch of the Central Command Vulcans that hadn't adapted to Sarek's teachings yet. That group itself could be an interesting ongoing plot, perhaps eventually finding they had more in common with the Romulans than they did their brethren on Vulcan.