I'm in the same situation, and finally decided to go with the Nezumi, which should help make the Dragonborn stick out a bit less.
Generally, it is a good idea to go with something that the players can easily imagine, so having lots of artwork available is quite useful. Especially if one of your reasons to change the race is so that people actually play it. Nezumi are strong in this area, because Rat-Men are a popular concept. In particular, you can find good art if you look for the Rokugan Nezumi, the Warhammer Skaven and the Kamigawa (Magic the Gathering) Nezumi.
Goblins are a good alternative, very iconic and rather beloved after Warcraft made them popular. For me, they didn't work because I intend to use Hobgoblins and Orcs heavily for the beginning of the campaign, but they are certainly a good choice.
Problems you might run into are that the Halfling abilities will most likely not fit your substitute race perfectly. Judging from "Races and Classes", Halfling's will have luck-based abilities. This can work for pretty much all small roguey races, but it's also not very specific. In particular, you would expect a bite/disease ability of Nezumi and excellent swimming of Otter/Frog-people. So a bit of rewriting might be necessary, but that is to be expected. If you go with Nezumi, you should probably look at what Wizards did for the Dragonborn's natural attacks, and give the Nezumi a slightly different copy.
