1. Why not make the Gladiator or Gladiator Aspect a unit that is swappable between heroes (just not stackable)? You could hire a specific Gladiator (with a unique ability) from the Tavern or Evil Shrine, maybe with an alignment circle like in H4 for what towns get what Gladiators. Gladiators would also have a hero and creature alignment preference. Some Gladiators would not be able to be traded and stay with the same hero, or once hired they stay with the same hero.
2. It would probably be best if Gladiators had to be hired specifically for all standard heroes, and heroes didn't start with them (especially if they use up a creature slot - see below). Of course any hero that is customized could also start with a custom Gladiator.
3. a) There could be defined consequences (standard/custom/scripted) if a Gladiator is down at the end of a battle, from permanent death of hero, to send hero to hero pool (probably accompanied by loss of army to prevent others hiring a big army), to permanent stat loss, or must revive the Gladiator for a certain cost out of battle.
3. b) I would suggest only to check if the Gladiator is still alive at the conclusion of the battle. To prevent cheese, maybe if the opposing hero runs or surrenders the remaining hero's Gladiator is automatically revived before the battle is officially ended (Second Chance, anyone?
3. c) Each Gladiator would have separate XP pool and skills (and maybe artifacts) and get the same experience as the hero gets for winning a battle - as long as they are standing at the conclusion.
There is an exception, see below.
4. Revive Gladiator combat spell is a strong possibility as well as some way to revive them out of combat, with a likely high cost. (But you will have to choose whether helping your Gladiator with your one spell per round is worth not helping your other troops.)
5. The system does not have to follow the WoG system, but stat boosts (including hitpoints) seem a certainty, as well as abilities that are in the game, and maybe new or H4 ones (some of the special abilities of H4 Magic skills would be worth considering) as well as their own spells. A unique ability for each specific Gladiator would be really worth having, and we would have no shortage of ideas that could be balanced properly.
6. Custom Gladiators for custom maps. As well as the option for a specific hero to start with a specific Standard Gladiator, you could also edit any aspect of the Gladiator including starting skills, stats, spells, abilities, unique ability and what requirements there are for auto-revive in battle (so the AI does not have to be taught to use any Revive Gladiator spell). This might also apply in certain cases to human-led Gladiators.
7. a) And one thing that hasn't been used in WoG: Multiple in the one army. For this to work, it could be that either if you have more than one Gladiator you use up creature slots, or you use one slot per Gladiator including the first one (giving a reason to go without other than possible penalties for the Gladiator being down at the end of a battle). And of course, the hero's winning experience would be divided by the number of Gladiators in the army that are standing at the end of the battle. So the hero would get a certain amount of experience, and that same experience would be also divided by the number of Gladiators standing at the end of the battle and added to each of their XP count.
7. b) Max. number of Gladiators/Gladiator Aspects per hero would be defined per map (including option of being turned off completely), but default would be 1.
7. c) This would allow Fellowship-type battles in the H2 Ironfist context. You could even allow more than five Gladiators if there was a way to define how they are placed on the H2 map - but probably only for custom maps (like LOTR maps for example).