I am not entirely for this sort of thing but it may be useful with a bit of tweaking...
Perhaps if, as a player levels up, they get a gold bonus (some percentage added) whenever they kill a mob that is within their own level range? (+/- 5 levels)
This might encourage players to stay in their own, level-appropriate zones more often rather than raping and pillaging in areas meant for the low levels.
I do think that having a small plat reward for certain level achievements is a good idea, but since this the only way that OTM generates revenue, I wouldn't hold my breath for it. A reasonable system for a plat reward would work like this IMO:
Reach level 10 - 5 free plat
Reach level 20 - 5 free plat
Reach level 30 - 5 free plat
Reach level 40 - 5 free plat
Reach level 50 - 10 free plat (and this continues until player reaches level 90)
Reach level 100 - 20 free plat (then back to 10 plat every 10 levels)
Even if the reward was 5 plat every ten levels it would be generous of the devs IMO, but for the reason stated, they may be reluctant to do this. After all, who will buy the cow if the milk is free?
On the other hand, this taste of platinum might even generate sales among some players who ordinarily wouldn't buy that sort of thing (assuming they get 'hooked').
The big downside of implementing this for the devs is that they would have to retroactively dole this out to all the players, lest they feel our wrath on the forums... That would probably recently in an immediate loss of revenue, which they don't want.
I don't care either way, the devs have been kind enough to give back 10% of plat buyers purchases not once but twice, and I for one don't see a major issue with plat buyers enjoying certain advantages over freemium users - so long as the free-to-play set are still able to have a decent game experience (which I feel they do have).