It seemed pretty obvious to me from the moment the system was announced that it was going to be comically flawed. What's sad is that it could be easily fixed if Paradox weren't so ridiculously fixated on arbitrary hard-number limits.
I would say that if the requirements were as follows, the system would not only work much better for GPs but probably make the diplomatic scene much more interesting in general;
A War can be Intervened into IF;
* The war has a lopsided force balance of greater than ~20%.
* The side with Greater Forces has achieved at least 10% Warscore, IE if they can start making demands.
* (Potentially) Alternatively to the above, you could have the War-Score or Force Disparity affect the two above limits. If a War is slanted heavily in favor of one side in terms of Power, then you don't need any positive War-Score for an intervention. If War-Score is heavily slanted towards one size, then you don't need a Force Disparity.
A Country can Intervene into a War that satisfies the above IF;
* They are a Great Power and the war involves at least 1 Great Power.
* They have a Rival on the Winning Side
* They are part of a Coalition that is targeting someone on the Winning Side
There you go. Those are the requirements. The implications of this are staggering, and greatly frees the warfare system to allow more dynamic war participation.
First; It immediately solves the current issue, because now it automatically takes into account the strength of Great Powers by basing the Intervenability of the war on this Strength rather than arbitrary number. If 4 Great Powers are losing against a single stronger Great Power then further interventions will happen on the side of the losing Great Powers, even though there's 4 GPs losing to only a single GP. Problem Solved.
Second; The Rival Intervention system, which I've suggested several times and it was extremely disappointing to see Paradox implement it in the worst way possible, allows these sorts of warfare escalations to happen at levels below the GP Stage. This is INCREDIBLY IMPORTANT because it means that now Alliances aren't necessarily the only way you can gain supporters for wars. One great thing would be if EUIV got the ability to Invite to War which Stellaris had, which could effectively be treated as Pre-Negotiated Interventions. So basically you could invite countries to your offensive wars, even if they aren't a direct Ally of your country, so long as they fulfill the requirements to Intervene into the war in question, obviously temporarily ignoring the "Must be above 10% Warscore" requirement, or being dependent on the Potentially More Complicated Criteria I suggest in the third bullet.
This could then allow the game to treat Alliances as a more important thing, making them rarer and harder to gain without completely shafting smaller nations who desperately need support in order to survive.
Third; This gives Coalitions another way of being a possible check on the power of Expansionistic Powers, by allowing them to Intervene. Since they woudn't turn the war into a Coalition War you will be able to Individually Peace them out. The biggest advantage is this would allow Coalitions to React to Wars While those Wars are Ongoing. This is the great current flaw of the Coalition System in my eyes, and in particular it means that Paradox has had to resort to comically ahistorical ridiculousness like "Can't Individual Peace Out a Coalition even when the event that Makes a Coalition explicitly mentions that the target of the Coalition survived by Individually Peacing Out the Coalition Members who then intervened on their side.
Think about that for a moment. In the current game, the Italian Wars are COMPLETELY IMPOSSIBLE, at least as far as getting them to happen the way they did historically. Under my above system; The Papal States could form a Coalition against Venice and draw in a bunch of powers. They declare war, a war which doesn't restrict them from peacing out individually because that's stupid. While in this war, France eventually signs a Peace with Venice and also annexes Milan because they were there. This gives them a great deal of Infamy and, because Venice doesn't actually have Infamy, Spain and Austria instead shift to being in a Coalition against France, and can now Intervene to stop them. And yeah I don't think that's the exact set of shifting alliances that happened but its still a hell of a lot better than the Farce that we have now.
Fourth; This means that the Interventions can be extremely variable. A war might start out as a 1v1 against equal powers and only draw in an Intervention after one side is decisively beaten. So long as both powers maintain a roughly equivalent Force Balance, no Intervention can happen. This, for the first time in any Paradox Game, actually Encourages you to fight Limited Wars rather than arbitrarily forcing you to take limited gains due to numerical limits that cannot be influenced at all. Now there's an actual Strategic Reason to avoid decisively crushing an opponent, because doing so would cause the war to escalate. On the other hand, if you feel confident that you can win even this escalated war, then feel free to push your luck. Imagine that. Strategic Choices in a Strategy Game.
A war could see a 1v1 devolve into a 2v3 brawl as powers enter and leave, escalating to a 1v5 once one of the powers manages to seriously start making gains, and then in the end finish off as a 1v78 as all of the minor powers start throwing their hats in behind the alliance trying to decisively crush this rapidly expanding force.
We might actually get wars that are Interesting!!