I agree there should be some solution to forgetting to select a kit. I do not think, however, that you should automatically select your last kit. There could be an even better system.
For me, personally, I never forget to select a kit. The only times when I play without a kit is if I join the game just before it starts, and that does happen fairly often.
I think, in the shop where you buy the kits, you should be able to set a default kit to select, and when you join a lobby, a green chat message will pop up "Selected kit [your default kit]", just as if you actually selected the kit manually.
Furthermore, for games where players can vote for game modes, it would be nice if you can select a separate default for each gamemode.
Of course, there could also be a different option in the kits shop that simply selects your last kit, which you can use instead.
An idea of how this could be implemented: In the kits shop, you can right click any kit you have already bought, and it will take you to another menu in which you can select what gamemodes this kit should be default for, and then exit. After you set a default, whenever you hover over the kit in the kits shop again, the lore on the item representing the kit will say "Selected as default for [gamemode]".
