Mid game join:
Stacking: LOL no. This would majorly upset the balance of arena and kill it even more. The about of RQing from the 40 bracket would be UNBELIEVEABLE. And it would hurt the nubsnubs to the point of no returnbecause it wouldnt be fun AT ALL (without some luck). Having a problem with teamwork usay? Well try talking to your team. Humans have this speical thing called communication, leting them recive help when needed. If they cant speek english pull up a translater or RQ or wish for more luck next game.
Locks: Meh, this one really depends on the bracket. It would work well for 40 but after that.. meh Would kill off the already limping 50, chance to finish 60 if they dont like WS and i dont think they could run EA, 70 would be decent with an EA/FA switch but WS would be a nogo.
MidGame: I would honestly love to see this one. As everyone knows, some brackets have a bit of an RQ problem. In arenas that are 5/5 this could have a big impact on your results! Poopy player or not they still could change the tide of battle. Not only that, then maybe people wont have to have there game cut short from RQers cus more people whould join before the game ends! Ive played a game were you can get banned for leaving a game (ofc that is way to harsh tho) But leaving directly affects the other players in the 5v5 games. Only problem with this i see is someone leaving on one char, then joining on another to get the edge. So maybe we can get a system that stops people from joining the same arena on that account (but that would need deves wouldnt it?)
If you want more arena i believe the key might live in the raits of Fortifacation and Oclay (at least in 40)