First I would like to say that I love the addition to team builder to the game, and consequently have played almost any different game mode since its release. I find the option to always play what I have in mind, and to my strengths to be great improvement on the old matchmaking model and I also enjoy the concept of hand crafting any team I want right down to putting deciding exactly what role in the game I wish for my team to fulfill. It's an amazing dynamic and I am very grateful for it. This said I do feel that there is a few areas that it could better fulfill.
Before I say exactly what it is I want to improve I first want to identify what it is that I feel are the underlying values that separate team builder from the traditional match making system.
-No arguing over lanes. Team builder solves for the anxiety of possibly not getting to play what it is you want to play, because some one has a good reason to pick it instead of you, and being a decent human means you should yield to them.
-Having more control of the champion on your team. Weather you're making the room or you're joining you have a lot more control over what champions doing what role it is you're going to be playing with. If you don't like something you have the il-refutable power to fix it.
-The ability to play what you want where you want. This is that classic "Break the meta" attitude that a lot of people don't appreciate. While I can "Break The Meta" in a normal game by calling ap kog-maw and locking in, this can have a very negative impact on my teammates. However in a team builder lobby you can pick that unconventional champion/role combo and it won't have a negative impact on your teammates at all, because the person who accepts you accepts that you will be playing that with them, and anyone who stays in the lobby accepts the same.
These are the three things that I find to be important when deciding which mode to play, and while I could also analyze the dis-advantages of picking it over the other, I don't want to focus on fixing that.
Now I'd argue that these advantages that draw people to team builder could be enhanced by a simple change. The issue currently is that there is a rather major flaw in the system that interferes with that "Build whatever team you want" fantasy. The current system pressures players to conform to the meta. Let's say that in my dream team I wanted to have me and my teammates do an all mid strategy. I would put mid lane for every role but if I didn't want to put all mages I would have to individually assign roles like a marksman, maybe a support, etc. If once I did that I started searching for players it would be nearly impossible to find a person that fulfilled your unusual requirements, and any players I did get that were willing to do an all mid strategy would almost certainly leave after a bit due to having to wait in the queue for possibly the full 1 hour you're allowed. The issue causing this lies in the way that solo players have to set up their specifications. Let's say I'm playing Caitlyn and I have to pick my lane. Am I going to put mid lane? Even if I was up to the idea of an all mid strategy I don't want to end up being a solo marksman mid, or maybe I don't want to increase my queue time into infinity. Or at least I don't want to enough that I would be willing to spend hours on something that probably wouldn't even work out. This is the issue. Players making a team cannot make any team they want because solo players are forced to be very specific in what they are willing to do, and if they do anything else besides the normal they are punished for it with increased queue times that will probably always out weigh the benefit of having a little extra fun dicking around for a game.
So as I said earlier there is an easy way to fix it. If you give players the option for solo queued players to pick their champion, and their role, but put the option for them to put "Any Lane" instead of having to pick one. This way the players are given the option to accept an unconventional team set up if the offer is presented to them, but don't have to suffer the consequences of committing to it. Immediately the only negative consequence I can think of is something along the lines of a mage, like Ziggs, ending up in the jungle because they wanted a Jungle "mage" but the person who is hosting the room would still have to decide if they wanted a jungle lb or not so it doesn't harm anything.
TL;DR: Currently it is to hard for players to get matched up in rooms that break the Meta, because the system heavily enforces the Meta. So make it so solo players can queue up as "Any Lane" and then all will be happy fun time in the land of Team Builder.