Agreed. I think the Romulans would be excellent big bads. How many people out there think that Future Guy and the Suliban were better? I think the Xindi had a unique origin and motivation, but they weren't so different that their motivation couldn't have been used for the Romulans. And the Romulans have more history and a weight behind them that none of ENT's homegrown antagonists did. And using the Romulans reinforces the idea that ENT was a prequel show, and a prequel show based shortly before the Earth-Romulan War. To not use the Romulans wouldn't have made much sense.
If you don't want to use them, do a prequel, post-Romulan War show. I actually would rather see that now, as another poster said, than a rebooted ENT. ENT had its shot, but so much of this time period was left unexplored by the show's untimely exit. But even if it had run it's course, it's doubtful that it would've focused much at all on the early years of the Federation.