You have lots of healthy, large plants in the background, and mid area...I really like the look of grasses or small bunchy plants in a foreground. Unfortunately, what I've found is that most of the grasses like medhigh-high lighting. However, you could try Echinodorus quadricostatus (Cuban broad leaf chain) as I think it's okay with med. lighting. Lots of the Cryptocorynes are good in lower light and would do just great with 2.5w/g. They come in a variety of colors (that salad bar thing is really interesting...do fish see in color? I guess so!), and are pretty easy keepers. Kind of finicky when it comes to alterations in their environment, though. They can experience 'crypt melt'...but they come around again when things settle down.
One other thing that I'd mention, just because I really love it, and it's sooooo easy to keep...java fern and or moss. And it stays fairly compact, which is good for foreground.
You know, for interest, you could go for a banana plant, or a lily bulb. The tubers are as interesting as the foliage.