Some definitions
Heroic: 1-20
Epic: 21-30
Divine: 20hd +30 lvl or something vague in that lines, rank 0..21+
In other words lvl 20 is still heroic.
Legendary is something DDO introduced.
Now that we clarified this let's talk quests.
Level 20 quests should have been heroic. Like the old ones and P. Carnival and sentinels should become lvl 21.
Lvl 20 quests should though give XP to lvl 20 toons like if epic quests. In simple terms: lvl 20quests should give more XP.
Level 18 quests played in elite become 20, what are these quests? This is where the problem lies... How you reward modified quests.
Personally I'm against the current level scaling for quests but I won't go to that discussion.
Lvl 20 quests should been the end game of heroics.
Now DDO has chosen that toons lvl 20 are epic, which doesn't really work out.
Similar treatment for level 30 quest and toons.
In practice max heroic XP reached should be 1xp less than level 21. And similar for epic XP.
And assuming legendary will reach up to 40, the same treatment.
Then they can do 41-50 for divine

but let's be frank PL are the divine ranks already

.
Okay enough with ideas that will never be implemented.
Based on the current situation:
Step 1:
A) Level 20 quests should be marked heroic
Or b) be if epic should become level 21.
Step 2:
Improve the XP of lvl 20 quests.
Step 3:
Use level 20 heroic together with lvl 30+ for some new adventure pack. Maybe a 4+1 chain pack.