I'm glad that sliders are gone and don't want to see them back, but there is a worthwhile point here.
EU4 already has lots of supposedly 'anti-blobbing' mechanics: overextension, AE, truce timers, revanchism and so on. However, as many players have noted, these are only obstacles to expansion and don't penalize blobs specifically. In practice, in the longer term they become blob protection mechanisms: a country that's worth several hundred % warscore is pretty much guaranteed to get several chances even when you totally dominate them, and every time you beat them, they get revanchism and you get a bunch of obstacles to deal with including 15-year truces. This is the case even if you are strictly in blob dismantling 'good guy' mode, forcing them to release countries or return cores rather than taking lots of land for yourself. (The game also makes life harder for blob dismantlers by giving very few CBs that justify releasing/returning.) By contrast, a sub-100% country only gets one chance to win, and even if they do somehow win against a much bigger country, they have to be careful: if they take a lot of land, the coalition danger is much greater than it would be for a large country (due to relative strength of alliances factoring into coalitions).
So the way things are now, as an existing blob you're pretty safe from external threats, much more so than as a small country. The only thing that can break you up quickly is a massive rebel wave, but you're not really at risk of that either unless you go over 100% overextension and so on. It would make sense if there was some sort of fragility modifier for countries with lots of unaccepted cultures or high average autonomy (due to e.g. having masses of territories and few states), and accepting cultures was a bit more difficult than it currently is. It doesn't have to be just rebels all the time. For instance you could also have increased stab costs, slower legitimacy growth, harder to gain absolutism, and so on; things that don't actively destroy you, but make it harder to get back on track if you mess up. Another thing I'd like to see is that your own fragility (however it's measured) translates to a multiplicative reduction in the warscore value of your land, and also a reduction in revanchism effectiveness, so it's easier for an enemy to take you apart if they win a decisive victory. This way, a Bahmanis that has blobbed all over India or whatever can be dismantled without unreasonable amounts of truces. Ongoing disasters, bankruptcy and other serious problems like that would increase your fragility, so there are good rewards available if you can jump on a big country at its moment of weakness (which is often how major conquests happened historically - the invader typically had some sort of inside backing).
On the subject of rebels using your military bonuses, that is indeed BS. Here's one way to make it more interesting:
- Most rebel types at the baseline don't get any special bonuses to morale, discipline, leader pips etc, but if they are sponsored, they get the best military bonuses out of their sponsors. Separatists are always considered to be sponsored by the tag they fight in the name of (even if it's dead), and also roll generals according to the army tradition of their tag.
This would make military-focused countries dangerous even in defeat, whereas a fat complacent blob with no military focus leads to correspondingly soft rebels.