Slitherine

Minor issue I noticed: I think the code checking for the Master and Servant achievement on Steam (as a client state, break your treaty with your protector). I got the achievement as Rome when I made Syracuse a client state, and integrated Syracuse's only province. Like I said, minor issue, but I thought I'd bring it up.

On that note: Some of the client state interactions seem odd. Playing as Rome with all of Italy and several provinces in Africa and Spain, with 3-4 armies of 100+ combat strength, and the bonus for "relative military strength" compared to 2-5 region nations is on the order of 5%. Which means military strength is basically irrelevant. The modifiers for "your legacy overshadows ours" is also odd--I would think that would make them want to attach themselves to a glorious nation, and as you reach the upper tiers of legacy it also makes it very hard to get any kind of client states. And possibly the most annoying one: "you own some of our objectives" or "we own and won't give up some of your objectives." The first of those two should reduce as the relative strength goes up--do you seriously intend to invade Rome with your little tribal army?--and the second, well, your options are to become a satellite or be invaded.

It just seems odd that Rome, which historically utilized client kingdoms heavily, basically cannot get the smallest of nations to agree to client status (and there is no way to install a client king--something I'm hoping becomes a thing as diplomacy gets looked at and expanded). Allies with 70+ opinion tend to have a 0%-15% acceptance chance, and of several offers Syracuse is the only one that actually accepted. Are others noticing the same sort of thing, or am I just bad at getting people to agree to be clients?

Playing as Rome also, 165 turns in with 3 client states (Aedui, Dalmatia, Dardania).
Started with Cooperation treaties and over time as the relationships improved concluded alliances with Aedui & Dardania. Dalmatia became a Client state in the low 30's and the other two once we hit 50 and 51 relationship.
Offered them gifts every 3-4 turns when we had common wars and stayed at war until its end with the elimination of our enemies checking every turn what the percentage for Client status was
and when it hit over 25% I would ask... sometimes the % dropped very low when rejected and it rose slowly with the gifts and shared wars.
Its easier now that they are clients and end the wars when I make peace but was an inconvenience when they were allies and consistently refused my overtures to end wars.
The only frustration I've had with them is the provinces they gift me. Sometimes good ones that completed a region (Dalmatia gave me Carnia and a province in Tisia that gave me full control of those regions) and more often bad ones that they're happy to get rid of - Aedui and I took out Helvetii which had most of the Alps provinces with Aedui conquering them and dumping them on me which in turn drove my decadence through the roof and dropped my CDR like a boulder.
Beginning to abandon them to their fates and should have started sooner.

The achievement given erroneously has been fixed for the upcoming 1.05 patch.

Allies and C/S will probably end giving regions when the new diplomacy system is in place, you'll have to ask nicely

Some modifiers are there as a counter weight to the tendency to snowball also.

Thanks Pocus, looking forward to the update.