It would be difficult to give titles out to a percentage of the player population - for example, with a 0.5% allocation of Gladiator titles, 1 player would get a title per every 200 players. That's unrealistic with our current population.
My opinion, for what it is worth, is to lay out the arena title rewards as the following:
#1 and #2 in US/EU get Title Gladiator. This allows for both members of a team to acquire the Title - or for opponents to attempt to steal away one of the Rank One spots without taking both (all or nothing).
#3 through #10 in each region get Gladiator. While this is a relatively larger number of players, (8 total), it reflects the competitiveness and skill that it takes to acquire a spot in the top 10. It also gives an incentive to players that are skilled enough to consistently prove themselves as truly excellent, but unable or unwilling to deal with the grind to secure the Title Gladiator spots.
#11 through #20 in each region get Duelist, which indicates some of the stronger players in each region. This isn't a very large gap in players from Gladiator (8 players versus 10 players), but any more players and the difference in skill between the highest and lowest rated players would start to make the title redundant.
#21 through #35 in each region would get Rival. Not much to say, except that the title starts to become more of a reward and less of an acknowledgment of skill.
#36 through #50 in each region would get Challenger. This would slot nicely into the current view of the Arena Ladder as the first page being the top 50 players.
Just my thoughts
I've put a good deal into thinking about how to distribute the End of Season rewards throughout a bracket with a limited population, and this is the best way I've been able to come up with.
Edit: Oh, and every class should get a reward for the highest rated player in that class. That's a genuinely good idea, especially in brackets where certain classes really struggle to climb past a certain point.
Edit 2: Here is the current rating required to achieve Title Gladiator, Gladiator, Duelist, Rival, and Challenger in each Region, as well as Global Class Awards, in the 19s bracket based on the metric I used above.
US
Title Gladiator: 2135
Gladiator: 2028
Duelist: 1918
Rival: 1884
Challenger: 1841
EU
Title Gladiator: 2204
Gladiator: 2012
Duelist: 1935
Rival: 1847
Challenger: 1723
Class Awards (as of 3/2/2016)
Druid: Stubs (rank 29 world) - EU - 1964
Hunter: Boomie (rank 45 world) - US - 1913
Mage: Wizkidone (rank 3 world) - US - 2142
Monk: Paysz (rank 37 world) - EU - 1935
Paladin: Katyperryirl (rank 4 world) - EU - 2135
Priest: Druhflynn (rank 4 world) - US - 2135
Rogue: Dodge (rank 22 world) - EU - 2010
Shaman: Stickerz (rank 1 world) - EU - 2218
Warlock: Faelqt (rank 191 world) - US - 1685
Warrior: Sannchrendx (rank 2 world) - EU - 2204