fukk it, let’s go Square Enix x Konami x Capcom.
Get them the sub job system, horizontal gear scaling (meaning some level 30 items are the strongest in the game like FFVI or FFXI) and 100+ playable character roster of all franchises from these companies. 5-6 member parties, 18-member alliance big battles.
Still keep character specific jobs but character should be able to have a ratio system like Capcom vs SNK 2. For example: Rydia should always be a main Summoner but maybe her best class is Summoner/Black Mage like FFIV but you can make her Summoner/Black Mage/White Mage like when she was a child where she may not have the best output damage, but she got some other utility.
That’s a sandbox and customizability right there. Would be able to min max like crazy.
Auron, Simon Belmont and Onimusha on the front line. Rydia, Primm and Rosa on the back line.