If they made the roulette bonus +5 levels. It wouldn't fix the issue. First you have to identify what the issue actually is. And I don't believe that has been done. Only the symptoms.
1. DPS have longer queue times
That's the 'issue' everyone brings up. What's the cause?
DPS is generally more fun? Maybe, some might not like the flavor of tanking.
Tanking requires a leadership role? Nope, that's a myth (friend of mine and I came back after a break since 2.4, they tank primarily, they've not taken a leadership role in any dungeon we haven't done before).
Healing is boring. Maybe, but there's no healer shortage (I'll wager healers are in a perfect spot right now).
What's the issue?
We need tanks in the leveling and PUG queues. But don't need them in endgame content. They're also pretty bad in solo content. This is the issue that plagues EVERY MMORPG with a random dungeon queue.
What happens is people don't level up tanks because you're not needed once you're max level and geared. Try it. Get into a static as a fresh new tank. Nope.. they've already got their tanks. They need DPS. But when leveling and gearing a DPS, you don't have tanks because no one needs tanks except those classes that are needed.
As you can see, its a Charlie Foxtrot of a Circle: The purpose of leveling tanks is to level needed roles. Who wants to volunteer to be eventually be obsolete? No one? That's what I figured.
So what's the fix? What's the solution?
1. You can require there to be more tanks in endgame. Make sure there are mechanics that require One Tank for every One Healer and Two DPS. Matching the leveling setup. Raids require HALF the tanks needed in non-raids. That's a problem.
or.
2. Flex size the groups. When DF is being used to form random groups. If the DPS queue exceeds 5 minutes. Allow the party finder to match parties of more than four players. Let it go to 5 (Tank, Healer, 3 DPS), or 6 (Tank, Healer, 4 DPS), or 7, or 8 (if its really really long). NPC/Boss health is increased by the percentage of players (double hp for 8 players), plus maybe 5-10%. This way the dungeons stay challenging and aren't simply steamrolled. Mechanics stay the same. This doesn't apply to premade parties, you can only go in as four and you don't get extras when you do (except if you queue as 2 or 3 people).
I prefer the second solution. Its more comprehensive. Its only con is it doesn't deal with a healer shortage, but I don't see that being an issue. it also sort of makes the dungeons easier. But since PUGs can complete all 4 man dungeons already with an above 90% success rate (if you see less than that.. sorry but it might be you). This is a non-issue anyway.
Rewarding double the bonus, triple. Maybe even 10x.. hell make it 100x. Or go stupid and give them 10 levels. All will not work. Congrats.. you're allowing someone to level a possibly obsolete role faster. That's not an incentive. It also does nothing for those who simply don't want to. One of my solutions does however. Why incentivize a role someone doesn't want to play if part of the problem, is possibly they just don't want to tank? Flexing the queues allows them to play the class they want to play. There's no good reason why a person who wants to play a Red Mage or Samurai ought not be able to play what they want.
In contrast, you could make a reward system work. Allow 'Adventurer in Need' to award all exp earned on the needed role to ALSO be applied to another Class/Job. So lets say I'm playing a Dark Knight and earn 400,000 exp total from a dungeon from all kills, bonuses, and end rewards. If the needed role was tank I earn 400,000 on the DRK as well as another job or class of my choosing. That's an incentive. The idea is to make the incentive juicy enough that everyone wants it. Everyone switches and DPS Shortage happens, so everyone goes back. Eventually the pools even out until no bonus is needed.
I still prefer the flex queue however. You still get players playing what they want to play in the end. Its a game, not employment.