I think having 10-20 submission at at time works great. In fact, if we go with a council, we can go through the list of pokemon (probably not one-by-one, as that would take ages, but maybe like in groups of 3 or something) and have a set period of time to talk/discuss these megas, and then a shorter time to vote on the best one. This ensures that we get more balanced megas.
As for the restricting rules, I don't see a problem with new abilities themselves, although I understand why people wouldn't want it. However, we should give new abilities scarcely, or make an ability that multiple mons can have. Not every mega needs a new ability. If we go with a council and have a voting process though, I don't see the need to make the rules stricter, as the council can easily filter out the obviously broken mons and can also make sure we don't get too many new abilities.
Also, when making new abilities, make sure it's either needed on the mon or it fits flavor-wise. Both would be preferred, but one or the other (flavor-based needs to make sure the ability isn't broken on the mon though) works fine.
As for the restricting rules, I don't see a problem with new abilities themselves, although I understand why people wouldn't want it. However, we should give new abilities scarcely, or make an ability that multiple mons can have. Not every mega needs a new ability. If we go with a council and have a voting process though, I don't see the need to make the rules stricter, as the council can easily filter out the obviously broken mons and can also make sure we don't get too many new abilities.
Also, when making new abilities, make sure it's either needed on the mon or it fits flavor-wise. Both would be preferred, but one or the other (flavor-based needs to make sure the ability isn't broken on the mon though) works fine.