Consider that the game is in a full blown rainbow meta. You should probably try and close gaps and fill holes in your teams first.
Investing to heavily into one element may end up making some content too hard.
Since you are not hl yet you might also want to wait before deciding on a gw 5* and sparking will change a lot in terms of team comps.
As for spark targets, they are only Limited or freshly released characters so you should probably look at Zooey, Lucio, Sturm or the summer limiteds if you can wait that long.
If you want more specific help, post your characters.