Improving tomato sauce in beef liver recipe
A few finely diced mushrooms or olives can add a lot of extra flavour, Worcester Sauce is of course an option too for quickly adding flavour. 

There are quite a few 'proper' recipes which recommend fried and blended celery but I've never found it to work that well, it can add body if thinly sliced but doesn't do much flavour wise.

Personally I'd add a good helping of coarse black pepper and some dried parsley. I've always found simply adding more garlic can add flavour too so you may want to experiment with that too.

If it's specifically more tomato flavour you want then really the only thing you can do is reduce it more.