The Avatar is one of the things that is least likely to change overall in the new codex. They may look at making the Wailing Doom do something, but for its points its an effective unit.
Aside from that, the Avatar is not a god. Its a tiny fragment of a warp entity manifested in the material universe, sustanined and summoned by the blood of its kin.
Its more a greater demon than a god.
I'd like to see the units which don't match their fluff tweaked. The Avatar is one of the H2H killers in the game, and has the WS to outclass anything else barring a Bloodthirster (appropriately).
The Shining Spears are a good example of a unit that is muddle of Close Combat fluff and Tank Hunting uses.
There is one potential that no one seems to ahve brought up.
We alredy have Codex: CWE, maybe they will do a proper Codex: Eldar, including Harlequins and Exodites as well, it would be kind of in line with their new Doctrines type Codices.
Taking Harlis as part of an Eldar force, not that brings back memories of 2nd edition.
An explanation and clarification of the Eldar associations diagram in the codex would be cool as well. Maybe with more fullf and updated history since Eldrad has perished.
In terms of rules, its likely to be a minor tweak if anything, for the most part the rules work, and once people know what they are doing the Eldar are a force to be feared.
But Exodites included again would be cool - I want my dragon riders back..