This is actually good news seeing someone take over and trying to manage these sort of events. Thumbs up on that!
(I think)There are two things preventing you and the community from participating into wargames at the moment :
1.Twink Cup - There are many teams (about 4-5, if not more) trying to get 10v10's every weekend, and I do not believe any of those would back down on a week of practice for a general 10v10. Twink Cup has more priority at the moment.
2.Merging 29s with 20s in a competitive 10v10 has always been rejected by players, and not without reason. From experience, i remember doing 10v10s and a 29 FC from the enemy team joined and our rogue/hunter Defense just could not end him because of the healing he was able to throw. The message here is more focused on having 10v10s on an equal and competitive level at the same time.
Tips :
1.When you get a lot of people interested to play, focus more on making them sign up with different characters. That gives you an opportunity to make this event even better, because it adds variety, and allows you to make different choices on who is playing what. That way, the event can be pleasant and sort of challenging to almost everyone.
2.Focus more on having team leaders on both the teams. A team leader must have experience in these sort of wargames, being able to instruct newer people into the games.
(Just my 2 cents aye).