Qssuming we see spellstrike as two actions with a single check for success/failure...
That would free up the third action for all builds and allow more round_to-round versatility.
It would also eliminate the need for slide casting and many other feats that only exist to fix the action economy issues created byb3 action striking spell.
This would also, perhaps counter-intuitively, make less need for those spells that exist only to make striking spell more attractive. All the extra riders and effects with striking spell would still be nice, but would no longer look like must-haves, instead allowing you to branch out more with feat choices.
I'd like to see the remaining sythesis reworked, and for each to add a unique reaction (something the class sorely lacks right now) or even a focus spell stance (a great idea others have suggested!). I'd prefer they feel less limiting to a weapon, or allow us to pick up an additional sythesis.
I'd love to see a new kensai style sythesis where the sword is part weapon, part familiar, part spellbook, maybe even part staff.
Feats to expand on the Tome concept, and the unarmed.
The 4 spell slots can work if we see more minor magic effects without spells (like the feats that add spellstrike riders, but not requiring spellstrike). Or a variation of the wave slots mentioned.
Even just 1 slot per level you've advanced out of would both open up utility for a class that is severely lacking it, and make a muliclads caster dedication a lot less appealing than it is currently.
Right now nearly all feats (while mostly interesting) are either built onto or rely on striking spell. Which is not satisfying when you run out of slots or don't have the action economy to use it.