• Better internet settings by default (cl_interp, etc. new players should be used to a rocket firing when they click)
• Matchmaking revamp. Remove graphic cvars, make it work with DX8, add a 2 class limit for all classes, add placement matches. Basically copy CSGO's matchmaking rank/elo system to an extent.
• Rebalance all weapons not on the whitelist. Balancing weapons for 6v6 does not make them less fun in 12v12. Yet balancing them for 12v12 does make 6v6 less fun to an alarming extent. (with no need for a whitelist, new players would have an easier time transitioning to competitive, and competitive players would have more weapons to explore unique strategies with.)
• Add a few custom huds as alternatives that you could pick from the options menu (rayshud, broeselhud, m0rehud, ahud.), along with being able to choose the size and color of your damage numbers, and maybe add a few more crosshairs and hitsounds. (remove the shock new players have when seeing competitive players games look completely different from their own.)
• Add the option to turn off ragdolls, blood, and explosion burns on the ground in the advanced video options menu. New players should not have to look to external resources to get atleast 60 fps in this game on a decent computer.
• Last but not least, add a few rocket jumping/airstrafing or movement tutorials. It is the most essential thing for new players to learn, and as it is now, most players wouldn't even understand where to begin or look to for help.
You've probably noticed a theme with these suggestions, and the theme is making the game we play, a lot easier to START playing in the first place. A lot of my friends played TF2 for a couple hundred hours, and then gave up, since they didn't have access to the same game I fell in love with. TF2 isn't lacking anything essential to be an amazing game that people love. It's just that the average player is used to everything being a LOT more accessible than TF2 currently is. If these changes were the TF2 team's priority, this game would be well on it's way to another 10 years. Although as it is now, who knows what we'll get.
(Also bringing in new players isn't a thing. Most people with a steam account have tried TF2 at some point or other. If they hear it got matchmaking and is really fun, a lot of them will come back and give it another try.)