This idea kinda reminds me of when people in WoW used to suggest that there should be extra rewards for Tanks and Healers in Dungeon/Raid Finder to help encourage people to queue as those roles and thus reduce wait times.
While those are good ideas in theory, the counter-point to it is that it's kind of a tricky, if not impossible, thing to really achieve.
If you make the reward TOO good, you get people who normally never play those roles playing them solely for the extra reward. Make it not enough of a reward and even the people who do main those roles won't be interested.
As it sits, no one wants to Support and Marksman. Why? I don't really know. To some I know it's supposedly boring even though this isn't past seasons where the Support was just a Wardbot. (protip folks: if you're wardbotting as a Support these days, you're doing it wrong.) Marksman....I'm not sure why people don't do that since at least there you're getting the farm and you have the possibility to carry the game. Honestly I can see why you wouldn't want to Support, but not wanting to Marksman is a bit odd to me. The only thing I can think of is that Marksman item-builds don't have a ton of variation.
Anyway....ultimately to improve Team Builder is not enough I think. I think the goal should be to improve matchmaking and roles in general. So to that end we have to ask ourselves: What is it about mid and top that are so appealing? Is it the diversity of champions and play-styles? Is it the diversity of item builds? Is it being in a solo-lane and thus not having to worry about another person affecting the matchup constantly? Is it the popularity of LCS mid and top players as opposed to supports and ADCs?
If the role can be improved, we can have more natural ADC/Marksman mains, as well as more Support mains. Then not only do we not have to have incentives to go to those roles, but we also have less people being unhappy because they are forced into those roles. There's always going to be personal preference, but for a majority it seems like there's something either missing in the experience or (in the case of supports) not communicated or believed.