My own instinct would be to change the rule for choosing spell effects:
"If you want, choose a second option from the list, as you wish, but then the spell returns to its native plasmic world unsettled."
Seems a little more interesting than the rule as-written (although also less effective for the caster, who otherwise has immense control over the shape the spell takes), creating some really interesting spell effects in a "yes, but" fashion. For instance, you cast "Becoming Miniscule", and roll two dice, with the results of "Instead of diminishing, you enlarge yourself to 5 meters in height", and "Nothing diminishes with you, only you yourself, naked".
Your choices are now, as before:
1. Instead of diminishing, you enlarge yourself.
2. You diminish, but without your belongings.
However, you can also create a combined effect:
3. If you are willing to unsettle the plasmid, you can: "diminish" a second person (but both naked, without either of your belongings), or diminish to 10 centimeters in height instead (still naked, without your belongings), or enlarge yourself and another.
A little more interesting than being able to choose ANY one option, feels quirky and unpredictable, and will create some interesting combinations (like two naked companions, in this case).
Another example:
You cast Screeching Stone Darts, and you hope they'll be silent instead of screeching (which you can accomplish by unsettling the plasmid). Let'say you're rolling two dice. You roll a 6 and a 1. So, either the spell is unsettled, or the darts attack at random. You're sneaking into a palace, and you CANNOT risk being discovered, so neither of those is good for you (the screeching would give you away in either case).
Your options are, if you're willing to unsettle the spell:
1. The spell is silent, as you wished, BUT the spell is unsettled twice (which means the tablet will no longer function; your spell is gone).
2. The spell is silent, as you wished, BUT it attacks targets at random. You can pray it works and doesn't kill you.
Seems like an interesting call to make! On one hand it makes spells less predictable and less powerful (as written, so long as you're willing to unsettle the spell, you can get exactly what you want), on the other hand it creates the possibility for the occasional "critical hit" at the cost of unsettling the spell (like summoning a creature which is obedient to you AND can speak in a human language).
Really not my place to be changing Vincent's rules, but I had this thought and decided (against my better judgement) to share it. At least that sounds like fun to me; I would try it.