Okay Dev's if your reading this one, I know it would take a LOT of work but I think it would amplify the game a lot
give anyone with access to spells a feat for spell synergy. When you use Spell Synergy you put two PREPARED SPELLS into it, and it comes out with a spell that is kind of like a combo of the two. Some spells wont have a synergy between others (for example maybe fireball + otilukes) but others could be really powerful (Wall of Fire + Firestorm) And others could maybe be bad for you and your party as well (Summon Monster IX + Greater Dispel Magic thus releasing it from captivity)
Not going to ask you for anything specific, and to balance it make it require a THIRD spell slot for wizards, clerics, artificers, and druids, but for FVS and sorc (because they can't do it like that) it would have to be done differently, I have no idea how you would do it.
With Meta Magics and DC's: DC's it uses the lowest DC spell used unless heightened then it goes from that spell for spell point cost (so if you throw a hypnotism on your hold monster then it is heightened as a lvl 1 spell but uses the higher lvl spell slot for your third) For empower/max/enlarge/ect you could make it charge: spell1 + spell2 + synergy + metamagic*1.5 + heighten (from lowest lvl spell) or use some different kind of math that would be hard for someone like me to comprehend.
How would this benefit the player? Okay I took Symbol of Death and synergized it with Heal. Now its a Symbol of Healing (or maybe it could be a symbol of Undeath)
Blade Barrier + Flame Turret could be a barrier of blades that spews out fire? like i said would be up to Dev's to figure it out, and if you want to help them out (if they chose it) put spell combo idea's down here ^_^