But the general idea for it is that it is a late game item which makes elves more practical since their is normally not a reason to pick elf over gnome.
Schadenfreude potion and any level-up catapulting strategy will favor elves over gnomes. I still hold that an item that only functions for one race is entirely inappropriate. Items should work for everyone, even if they may be better suited for specific races.
Maybe it could be +X mana from outside sources, not including items
This would be completely broken on its own. With JJ, extra mana boosters, and a fairly tame conversion loadout on an elf you could build up around +15 bonus mana with ease.
By comparison the mage plate is +1 MP / -5% attack bonus every two levels (maxing out at +5 MP for -25% attack bonus) and the elven boots are +3 MP / +15% magic resist. Quite simply such a mana-boosting effect would be unprecedentedly powerful, and would require drastic downsides to be balanced. Like, probably disabling mana regeneration altogether.
Secondly, effects that explicitly magnify other bonuses are unheardof in Desktop Dungeons. The closest is the interaction between base attack and attack bonus, but that's a fundamental game mechanic. In addition to probably requiring a complete overhaul of deity balance, it probably has way too many abusive applications.
While it is true you could rush this item to be proc, but you probably would be drained early game resources, which is a fair trade off for how I imaged the item.
Hardly; the benefits accrued from double mana regeneration would easily compensate for the loss of a preferred prep.
This should increase the characters who could use it
Yup, now it's gamebreaking for everybody