The only way to have more elements, is to have a higher grade doll. 

[Doll Grade] = [Number of elements] 

1-star doll = 3 

2-star doll = 4

3-star doll = 5

4-star doll = 6

You can get a 5-star doll as a collection reward. 

If you have a lot of doll materials, grab those sliders all the way up—until MAX. That way, you'll have a higher chance to craft higher grade dolls. This will need 400 doll materials. 

If you don't have enough doll materials, then save up! 

If you don't have enough dolls to bring to expedition however, you should craft more dolls no matter if they are only 1 or 2-grade. It's best to have a balanced team, before you aim higher! 

I hope this helps! ♪(๑ᴖ◡ᴖ๑)♪

Don't worry if you don't meet the element conditions! That just means you can't leave them idle. You have to help tap and swipe so you can get all the rewards. 

If you want to focus on a certain element, like fire, the random charms will really help. The only problem is... they seem to have removed the "instill energy" option in favor for the "transfer energy". 

I'm currently protesting about that! ๑`^´๑

I almost forgot! You can get higher grade dolls through promotion too! 

Once your doll is max level, you will need gold and dolls of the same grade in order to promote it to a higher grade. 

The dolls you need to sacrifice depends on the grade of the doll you are promoting. If the doll is 1-star, you need to sacrifice 1 doll. If the doll is 2-star, you have to sacrifice 2. It goes like that. ヾ(◍´▿`◍)ﾉﾞ