Forget it. A runaway leader problem would be created because completed tunnels can't be touched. Whether they were originally risky, high-scoring tunnels or not, there comes a point where one person might have so many points that nobody else can catch up.
I'd rather not add a layer of complexity that would allow completed tunnels to collapse. In addition to being confusing, it would probably make players question why tunnels should be completed in the first place.
Instead, I think I'll stick with the single tunnel system from GrimTunnel. Players can already press their luck. It's just that it happens with opposing tunnels instead of their own. Players have to decide whether to place high point cards that can be struck down at once or low point cards that make earlier segments harder to collapse. Then they have to hope the other players don't mess it up.
In addition, the math for deciding battle results has been simplified. First, double power cards have been removed. Each card is either positive, negative or ignored. Second, only one faction will be played at a time. Players only have to read a single value per card and each side has only one faction of support cards.
I then added a brawler flag to cards with one positive and two negative results. Previously, these cards were nice to shove into an opponent's support, but it was risky to play them as one's own invaders/guards. The brawler flag gives them more value by allowing them to be played in addition to an invader/guard. This also creates a method for players to place more than one power point at a time now that the double power cards are gone.
Meanwhile, I've removed the card transfer effects that were on some cards. Most of the game involves passing cards through support so I don't see the point.
There's now less variation in my cards. I'm debating whether I should include multiple copies of some in each deck. Each player places roughly 5 cards per round, but some of them are reclaimed. I'm not yet sure how many are required for a game to last just long enough to reward planning... It's probably a good thing I'm creating a digital version first.
On the bright side, the game itself seems more solid.