I really like F2K - I actually was playing it on Pinball Arcade today and got a new high with 3,570,610.

The original ruleset favors completing the MB once to light specials and some additional scoring, but then it peters out after that and you're left just shooting the spinner instead of exploring the table further. So my focus would be on expanding the role of MB beyond the one-time lights. The alphabet lanes are the essence of the original ruleset and I wouldn't mess with them, rather, I would focus on making them stay in the role of being the main bottleneck, because they keep your eyes moving around the table looking for more opportunities. It's a great treasure-hunt type of gameplay. Adding too much emphasis elsewhere in the playfield is either going to make it a single shot game(spinner/drops/center) or too wild(anything around the pops).

One thing I would do, though, rather than lane changes, is to spot the last letter, once it's down to the final one, by hitting the center stand-up target. What usually happens when it's a struggle to finish BLASTOFF is that there is one annoyingly difficult lane that just can't be hit, but it's a different one every time. It's good to have an alternative available to smooth things out, and there's some precedent for this in the existing stand-up targets at the top of the playfield and the way the center target works after starting MB.

Other ideas for scoring:

A bonus collect jackpot and light show for 5 hits on the center stand-up target during MB - a good reuse of the 5-4-3-2-1 voice countdown in a different form and it makes hitting that target feel more "final" than the current thing, where you can just hit it once by chance and get your special. If not challenging enough, make it a hurry-up too.

Progress a super multiplier, which multiplies on top of the regular one, for completing the launch sequence, so that repeatedly going from MB to launch will give you the biggest bonus. Then ending MB by accident is not a "gotcha" - you're still progressing in the game even if you missed an opportunity for jackpot.

Add a hurry-up for the countdown drop targets: instead of a fixed score each time you progress in the sequence, it adds progressive score but also starts counting down a 15 second reset timer, so if you fire away to do your 5-4-3-2-1 and survive, the risk/reward balances out.