Tips for a more flavorful tempura batter?!
1/2 c flour
1/2 c corn starch
1 tsp baking soda
1 tsp baking powder
1 tsp sugar
1/2 tsp salt
2/3c ice cold water
1 egg.
Combine dry ingredients in separate bowl. Beat egg into water. Add dry ingredients to egg water mixture. Stir together until mixed but lumpy. Dip item to be battered in mix to coat completely on all sides (coating will appear thin but will expand in oil). Immediately drop battered from batter bowl into oil. Fry until golden brown.
Answers:
Tempura batter is generally fairly bland. The key to truly great tempura is VERY cold batter, so when it hits the hot oil, it creates lots of sharp shards of crispiness. This is achieved by putting a few ice cubes in your batter, and a little bubbly soda/seltzer water helps, too. If you want more flavor, do what the restaurants do: salt them as soon as they're out of the oil. The Japanese get most of their flavor in their tempura in the dipping sauce (tentsuyu). Recipes online.
Tempura batter never has much flavor, that's what the dipping sauces are for. With such a thin batter, it is best not to add spices. It makes the batter break apart. That's why tempura batters are plain.
I would start by increasing the salt content to at least match the sugar content, or maybe even an additional teaspoon (1 1/2). If you want to add some spiciness, add a pinch of cayenne. If you're frying sweet things, add a pinch of cinnamon. Ground ginger would be a nice compliment to the batter if you're doing an Asian theme.
As far as the oil, you could use peanut oil which has a high smoke point and would lend its flavor to the batter.
An additional tip I have is to replace the ice cold water with ice cold club soda. The club soda will give you an infinitely lighter batter.