As we all know, at the moment the Diplomatic options are few - understandable, given the importance of working on the core game systems rather than chrome.
However, perhaps some ideas can be generated for additional options - and I have one suggestion:
In historical reality, the south German states' commitment to the war and their military resources was limited - after the initial contingents represented in the game, I can find no reference to additional regiments "sent to the front".
However, with both their historical patrons (Austria and France) involved, with the legal sanction of the Empire against Frederick, and given the "religious war" dimension of the 7YW which has lately attracted renewed scholarly attention, the rulers of Wuerttemberg and Bavaria did not lack for incentives to strain their resources to provide additional troops, particularly if subsidies had been forthcoming from the major combatants. So, there is sufficient historical justification for options providing additional contingents from these two states.
In game terms (specifically, the 56-63 campaign I'm playing with Bertram and a series of solitaire tests) it's not usual for both contingents to be used up by 1759 or so, if they've been in combat. My observation is that both Wur and Bav end up with large pools of replacements which can't be used because their regiments have been destroyed.
So I propose diplo options similar to the Swedish reinforcements or the periodic contingents of Ger/Swi/Walloon troops the French receive - a couple of bdes in return for (say) 10EP and $25.