Yep. Claim system could be useful. Each nation chooses a set of priority in order. All of war score earned against one nation will distribute in the most interest province in that nation first, then the second interest province then the third one and so on. When a province is full of war score in it, that province will automatically be transferred to the suitable owner and stay with him in the peace deal. Kinda similar to the deposit of war score. If the desire land overlaps, the nation who fills the score for that province first earns it. It depends on the priority and the score you earned from the specific enemy nation only. So no more screw up peace conference.
For example, Italy sat provinces of vital interest: 1. Dalmatia 2. Slovenia 3. Montenegro
Germany: 1. Slovenia 2. Serbia
Hungary: 1. Vojvodina 2. West Banat
Romania: 1. West Banat
Italy declared war on Yugoslavia and called in Germany and Hungary. Yugoslavia then joined the Allies and WW2 began.
The war score points nations earned would be deposited in provinces of interest in order of priority.
Italy fulfilled the cost for Dalmatia first. Therefore, Dalmatia was automatically transferred to Italy from German occupation and stayed with Italy in the peace conference.
Germany fully scored for Slovenia before Italy so Slovenia belonged to Germany. The war score points for Slovenia of Italy would instead be transferred to the province of Montenegro as the third province of interest.
Hungary fully got both Vojvodina and West Banat.
Romania wanted to join the war but his only province of vital interest was by Hungary.
This was when the mutual agreement mechanic kicked in to solve the problem.
Romania proposed to Hungary to join the war if he was given West Banat from the owner Hungary.
If Hungary agreed, West Banat went to Romania. Hungary got back the points deposited in West Banat in addition to the points paid by Romania for the cost of West Banat. Which meant Hungary got double points out of West Banat. This was the mutual agreement between Hungary and Romania.
But Hungary refused the proposal.
Romania then approached the faction leader Germany and proposed to join the war in exchange for West Banat from faction member Hungary. Germany agreed and sent message to Hungary to give Romania if he did not want to be kicked out of the faction.
Hungary could agree and got the double points from West Banat. But Hungary had something else on his mind.
He then sat new interest in the province of Serbia from Germany. After that, he sent a new proposal to Germany to give him the province of Serbia in exchange for West Banat.
Germany agreed and signed a mutual agreement with Hungary. As a result, Serbia was transferred to Hungary while West Banat was to Germany.
Germany then signed a new agreement and gave West Banat to Romania in exchange for him joining in the war.
Problem solved. Everyone got what they wanted. No more frustrating peace deal anymore. Even the joining late nations in faction and war could get something in return. No one works for free anyway.
WW2 is also known for the importance in diplomacy outside of military campaign. A diplomat could screw the chance for Spain joining the war. Historically, Hitler proposed to give as much land in Ukraine as Antonescu wanted, but he refused in the hope of getting Transylvania back. Hitler also promised to give land to Hungary and Bulgaria in return for them joining the war.
Therefore, claiming vital of interest and mutual agreement are must-have mechanics.