Nice call on [XXX] Synergy - I was actually considering using the word Synergy in the name yesterday at my family gathering (for the exact same reasons).
I don't think the level of groupings will get too insane - the big potential "problem areas" will be chance and style in my mind, and certainly not spellcasting (+3-5 DC, at most, isn't a huge drama when you think about it - especially given a fairly high level of investment and a specialised build/concept).
Multiclass casters (with Swashbuckler, Gallant requires way to much investment) will be 2 spell levels behind their compatriates, for Chance + Style + Melee to be added to their DC, which is cool - but 2 extra spell levels is far more useful, IMO. Also severely limits their origins choices to (likely) a single option. On the other hand [Combat] -> Spellcasting would make for a cool battlemage archetype, and be useful for the specialists (Chaneller with [Combat] Synergy would be delicious, but not overpowered).
The only thing I would never do is a [XXX] -> Chance ability. 4-5 Extra Action Dice (especially if [XXX] is easy to obtain - any combat tree for example) is just too much. Take that opinion with a grain of salt though, because I also think that Chance Expert is undercosted by a large margin (and to be honest, I have a well documented problem with Fortunate).
I think we should make a new page in the wiki for improvised Origin abilities - there's a heap of them floating around on different threads, and having them in one place would be good (easier to find, too). I'm busy for the rest of today, but I'll do it tonight / tomorrow if I get a chance and no one else does it.