I know, I'm breaking a rule by suggesting unreleased content. Down to business now.
Sparks of Life are going to be used to revive. However, in beginning missions a player will have no chance of reviving unless given a SoL (as they won't have much of a chance to gain a SoL previously otherwise), and even then they'll most likely use them up quickly. This leads to frustration at not being able to revive. Thus, a new player will most likely quit.
Sparks of Life should be categorized by Tier. T1 SoLs should only be used in T1, T2 SoLs in T2, etc. T1 missions would have a higher chance of dropping SoLs, whereas T3 missions will have lower chances. Why? In the beginning, the player will have many chances to revive, thus enjoying the game. As the game progresses, they will gradually receive less SoLs but by then they'll already understand and enjoy the game and keep playing. This will also increase T3 difficulty in comparison to T1.
No. we don't need another way to rev, this issue is already dealt with the fact earlier tire revs start out cheaper for revs. This way in tire 1 if you have to rev you often make the mist back that you ended up spending before you get 2 more levels done. I get the idea here but it really isn't needed.