In heroics, turning can be very effective. It does taper off slightly at high heroic levels, but it is in epic levels that it falls flat. I know that clerics may be getting a pass soon(tm), but I do not believe that the fix can be put into cleric. Boosting turns in the cleric realm enough to make it effective in epic would make it ridiculous in heroic. That said, I think some alterations to the background mechanics and epic feats are the places to look.
From the wiki:
1. Turning check: A charisma check (1d20 + your charisma modifier) determines the hit dice of the most powerful enemy you can affect (See table below.)
2. Turning damage: Total number of hit dice you can affect is 2d6 + effective cleric level + charisma modifier (Paladins get a -3 to level.)
Turning value is the effective cleric level.
Turning check result Highest hit dice turned
up to 0 Turning value - 4
1-3 Turning value - 3
4-6 Turning value - 2
7-9 Turning value - 1
10-12 Turning value
13-15 Turning value + 1
16-18 Turning value + 2
19-21 Turning value + 3
22+ Turning value + 4
Okay. One of the first things to notice is that the turn check table stops very early for epics. So that leads to:
Suggestion #1. Continue the table upwards. I would even consider doing it at intervals of 2 instead of 3. So,
22/23 = +4
24/25 = +5
26/27 = +6 etc. A stat score of 50+ is not unusual in epics these days. Continue the table up to 50. (That is 1d20+30. Higher that most would ever reach, but would prevent running out of the top of the table.)
Suggestion #2. Epic levels should add to cleric level/Max HD/Total HD.
(I looked around again because I thought it had be discussed before. It might be happening, but I cannot find a definitive reference. If it is not already happening, this would help.)
Suggestion #3. +[W] mechanic. Start with W=1. Multiply the Turn check, Max HD and Total HD results by W.
This provides a mechanic by which feats and gear could be more easily added in epics.
Suggestion #4. Epic feats to boost turning. (When you look at epic feats you can find ones that add damage, crit range, spell power, spell crits etc, but not for turning.)
-- Epic Turning. Requires CHA 21. Adds +1W
-- Greater Epic Turning. Adds +1W
(This numbers can, of course, be adjusted if they are over/under powered.)
(The first feat may also be level restricted to min level 24 if it creates too much boost at 21. etc.)
Suggestion #5. Add epic gear with Turn +W values.
All of this preserves turning unchanged in heroics. It also keeps it all available to paladins (should they want it.).
This provides a way to be awesome at turning, but comes at a cost. (Epics feats are valuable and so are gear slots.)
These should be relatively easy adjustments. i.e. expanding an already existing table. Adding a multiplier to an existing formula. (I hope. )
Finally, these could be put in completely separate from the cleric pass.
For reference:
https://www.ddo.com/forums/showthread.php/367872
http://ddowiki.com/page/Turn_Undead