I think that for now, goals should be easy to understand, easy to figure out how to achieve [at least one way], and easy to visualize on the playfield and on viewers' screens.
Excellent examples would be to use AFM and MB:
For AFM, goals would be:
1. start Total Annihilation
2. collect Lock Multiball Superjackpot
3. Most Martians killed in 3 minutes [3 would work better than 2 here I think]
4. Collect two cities
5. 2- or 3-minute drill [longer in later rounds]
For MB, the 4 non-score goals would be to collect instruments. Maybe use Werewolf, Dracula, Frankie and Creature for basic level and save Bride and Mummy for later rounds? [Bride is probably the most challenging to finish; Mummy takes the longest to start].
For both of these machines, everything is clear and easy to see as you progress. They're both player- and viewer-friendly.
I would avoid using games like JY or JM where it's harder for people who haven't played them much to figure out what the heck is going on. Frankly, the latest couple of Sterns aren't the best choices, either - - they're too complicated in some ways, getting away from "normal" pinball and into a kind of pinball - logic switch - video hybrid with all of the shot multipliers and such. Harder for non-experts to follow, take much longer to learn [you didn't have to _study_ Paragon, Space Shuttle, AFM or TAF that long to figure out what to do].
Mode-based games like TAF, TFTC, Tommy, etc. are somewhat problematic. "Start N modes" is one way to go, but that's just one goal. Completing a specific mode is a problem since you can't control which one starts. H2H will surely work for some of them, but the goal setting will be trickier.
In some ways, the old "Pin-Tac-Toe" goals we used in FSPA would work in many situations.
Regardless of one's opinion of my game comments, it would be good to compile a list of machines which lend themselves to good sets of goals and give a rationale for why they're good choices. For instance, The AFM and MB examples above, besides being pretty clear, all require about 10-15 specific shots to be made; the better goals used last weekend, e.g. start Elevator MB, get a specific 2X, start Toybox Level N, etc., all seemed to be in this same range, too.
As far as "straightforward" vs. "open-ended," as long as the viewers can follow what's happening and the mid-level players can figure out at least one way to get there, either is probably okay.
I'll be giving thought to specific machines that lend themselves to "good goal playing and viewing" and post more some time next week.