I would like to suggest giving pure caster classes, such as Wizards, Druids, and Clerics, a substantial DC bonus to spells:
Because they'd give up on important defensive skills that they normally get from multiclassing. Pure casters sacrifice versatility and utility that come with multiclassing, so a DC bonus would help them maintain effectiveness in their specialized roles.
The current character building meta focuses far too much on Constitution rather than what an Epic Caster would realistically thrive for—gaining even deeper expertise in their caster classes instead of "bulking up." A DC bonus would encourage a more thematically, roleplay appropriate focus on magical mastery over physical durability - which is a fair double edged sword.
Because many DC-based spells are quite weak in the endgame, even for level 9 spells, compared to the resistances you'd face in PvE and PvP. Additionally, many classes simply can't afford to cast the same spells repeatedly to make use of the current DC overcoming mechanic with the exception of the Evoker. For example Clerics have a poor or no refund chance on higher-tier spells beyond a certain spell level.
PvP is, regardless of what the intentions are set for, a quite big part of the server, especially due to the conflicts that naturally arise from the setting of the Underdark (exiles). From my NWN experience back when I was a kid, as most RP servers eventually died out, many of the high-quality PvP servers—often the only ones that remained active in the end—implemented adjustments like this to ensure all caster classes remained viable