I was thinking of this the other day - much the same as the ceramic ball in TZ..
This is right "out there" - but how good would it be if the BALL became interactive with the player ? What about a different colored ball for each Thunderbird ?
So you would have a green ball for Thunderbird 2
A yellow ball for Thunderbird 4
You achieve the "mission" where you need T2 and T4 - an underwater episode - Once the mission starts, you get a 2 ball multiball - one ball GREEN and one ball YELLOW. With these 2 balls, you need to save the day ! Whether that be a set of in-line targets, or drop targets..
Another idea - Why can't Thunderbird 4 be under a mini playfield, since it is always underwater ? Much like Streetfighter II and the car ?