If we speaks about little stats increase(ie +2 to everything from posts above), thats just 10-20 extra damage for 5-10 reborns. And extra 1-2ac. Good, but doesnt seems like any gamebreaking addition.
If we speaks about prime skills transfer, which is way more interesting in terms of gameplay.
Max one player can get is:
- warrior: parry, second wind, riposte, massive critical hit
- monk: focus, presage,wraithtouch, kensai, prediction, deathtouch
- bard: adaptation to circumstances, dance with the dead, dance of the seven luck gods, call of the sidhe, - knights blessing, song of the mist, knights prayer
- thief: circle, apply poison, sleight of hand
- druid: shapeshift form to form, shield of thorns, canopied arbor
- necromancer: ritual of resurrection, wraithform, death pact
- mage: frostbolt, reflection, mirror image, blizzard
- cleric: dark mace, aerial servant, circle of healing, kylas delight
(Btw we dont have prime druids, necros and cleric avatar abilities. Maybe worth to think about it and add something. For druids that could be extra reduce lag and costs of shifts for example. For clerics and necros - auto assist of pets of their types. But thats just as sideline of topic.)
Adding power is something, that could be used at same time with other abilities. Otherwise its just diversity.
- So warriors skills are good. All usefull and strong. Common choice to pick for second class probably for non-warriors and non-druids

- For those, who grips focus is good. But not any op. Deathtouch is cool, but not real power difference between it or good thief's stab with poison. Presage gives some utility, but not real power.
- I am not good with bards, but it seems that no real power in any of those. Support things mostly. Good to have, nothing to fear about.
- Circle is just another incombat ability, no power. Poison and sleight of hand complect is good addition for stabbers. Not seems as overkill. All thiefs abilities even adding some equality to monk/thiefs choice of combat oppeners. Atm monk seems more common and powerfull variant.
-Druid gives only shield of torns. Quite strong. With current extra regen that still worth to pick it second class.
- Necro giving juju and wraithform. That one really powerfull probably. Though juju is limited by basic non-necro prime order issues, but still impressive power gain.
-Mage gives only mirrors actually. And they are weak atm, too costly, too low effect. Frost is little better then wind, but only for blasters(and thats maybe bards and clerics only?). Who are limited by spell dam cap anyway to be compared to real mage power.
-Clerics gives servant and mace. Servant is solid addition because its passive. Mace is fairly balanced imho.
So some final coombination will be
Parry, riposte, massive crit, shield of torns, wraithform and aerial servant as passive effects. And juju. With downside effect of more mana consumption for keeping all new spells up.
All active abilities doesnt seem to me as overkill, because they all not will be better then prime ones. So better stab or deathtouch. Frostbolts instead of winds. And second wind. Thats probably all?
Which will take several years of constant intensive playing, dont forget.
Lets think what one player able to do with such power.
Solo Lucifer and other epics? Some ppl able to do it already. Takes plenty of time. Will be little faster. Maybe even twice faster. Instead of 12 hours for Lucifer, will be 6

Worth to do if you need balls of Lucifer, or any other pop from epics.
But doubtfully for any other reason. Too long. And still will be too long after 8xreborns i guess.
We not adding that much power to splork epics.
And most ppl still prefer to group, because its more interesting and actually easy.
Group of such avatars will kill things way faster for sure. But remember, that most of them will be in rebirth process constantly for next several years

And on other hand, that will allow to add new hard areas. With increased power. And increased output though. New hard forges, new hard epics

Such additions will never ruin the mud in my opinion.