In Aow:SM you could watch other tactical battles (although I thought this was kind of cheating at least it kept waiting players a bit entertained). I have a number of ideas that could create better flow for players.
1. Limit rush turn taking tactics so players that make their moves quickly dont get an advantage. (As in attacking a city first before another player).
2. Make tactical battles fun for other players either by 1 letting them watch or 2 by allowing them to take their next turn while the players slog it out allowing for catch up from the players once they finish the battle. Potentially if its a long battle maybe the players fighting in tactical can allow players to move on in their turns (if for instance no players are that near each other to influence the 2 battling). All new aggressive moves towards the 2 players who are in tactical would be frozen allowing the players to react to the replay of turns taken before the new aggression when they resume their normal play after tactical.
3. Like total war, only use tactical when 2 sides are of equal(ish) power otherwise autoresolve. This could be on a sliding scale. (From fight only on 40-60% range through to 10-100% range depending on what people like). Also maybe a vote on tactical as well rather than a threshold system or a combination of the two? So one player wants to auto the other might vote to tactical the other players can veto this.
Just some ideas.