What if, while a Magus is charged with a Striking Spell, and successfully hits with a physical strike. They roll a spell strike using the same MAP as the physical strike (be it Agile, etc.).
The difference being give the Magus has a new Free action (with the following trigger):
Succeeding with an physical attack during a Striking Spell, but getting a simple failure on the spell's attack roll.
Effect: The magus holds the charge, of the Striking Spell. The spell attack counts towards their future MAP as an attack, but the Striking Spell is still prepared for a subsequent strikes. (this does not extend the timeframe past the end of the round after the spell was cast however)
This would mean that the Magus, when using spell strike would be trading the ability to make a spell attack only dependent on one roll, to one being dependent on a prerequisite physical strike hit, but would allow them several attempt to get the combination of of hits on both they physical and subsequent spell strikes to land the spell. It think this ability to re-roll misses in effect would give more of a reason for the ranged Magus to use this ability, rather than simply casting the ranged spells outside the Striking Spell envelope during the lower levels.
This would not stop them from ever losing a spell, as during a spell attack roll if they got a critical failure, but gives them a chance to try again on simple failures. And yes, for some reason something very similar to this was basically listed as an 18th level feat(second hand strike) although that covered crit saved saves, immunity to the spell, etc. as well. But is this really that giant a boost (or more importantly unbalanced)? It doesn't seem like it to me, but on the other hand it might be enough to make spell strikes give an extra edge, even if on average the magus might miss on more spell attack rolls than your average spellcaster. It would also help conserve the four spell slots they have, helping them to get a higher percentage of them to stick, than their raw spell attack roll proficiency numbers would normally allow for.
If this was viewed as too powerful an ability, it could either be turned into a reaction, which would basically make it only usable once per turn and would eat up other potential uses of your reaction.
Another option for limiting it would be to have it be that if it has been used on a particular cast spell to enable another attempt, it could be that any critical on subsequent physical attacks, might not provide the normal boost of effect for this spells instance.
Again, the Magus would be taking longer to actually hit with their spell strikes, since they are taking their actions to cast the spells, and taking actions to make strikes, and only they strikes that hit become chances to roll their spell strikes, and when those miss, they have to make more physical strikes attempts to hit, to trigger yet another spell strike, potentially missing from not landing another blow. I.e. Magus's have a tendency to miss more overall, but the misses are less impact to their spell-casting resources, and they keep swinging, and it isn't uncommon for them to get some of the more spectacular results from the diligence.
I.e. they wouldn't really be known particularly for their accuracy, without potentially magical aid, but would be known for their perseverance and periodic results.
This I imagined only affected spell attack rolls, but if people were concerned about spells where the target makes saving throws, you could have the Hold the Charge be an 'option' if the target makes a simple success on the saving throw. (they would still lose the charge on a critical success, and the Magus could choose, for instance to accept the half-damage, or hold the charge, hoping to get more next time) Personally, I'm inclined to stick with it affecting only the spell attack spells, but this would be an possible take.