I know this very much comes down to personal choice and liking the models. But since I had the island of blood set my first choice would be High Elves and Skaven. Both seem pretty different, High Elves are fast, Skaven are hella random. Skaven seem like the way different race to play but the island of blood + skaven battalion gives me 80 clan rats and 4 ogres plus a little extra High Elves island + battalion gives 20 spears, 20 archers, 10 swordmasters, 5 reavers, 10 seaguard
I picked up a IoB and choose Skaven, which was great cause I knew a guy who picked up IoB for the Helves, a switcheroo later and I had a ton of clan rats/slaves four ROgres and I never looked back. The skaven are a really fun army and they have some very nice models, the only downside is that you need lots of models, and by lots I mean I have a 2250 point list with 200 models in it, so one needs to have motivation to paint the army.