Reserves (like in new King's Bounty games) and/or reinforcements during battle

Reserves are troops that you can swap in if you have a vacant slot when not in battle, but if you lose you lose all your reserves too (and you can't swap them in while in battle). Reinforcements could be implemented as scripts and/or the ability to bring in Reserves into the current battle under certain conditions.

Not sure if I support that because it will mean a total overhaul of H2 gameplay with not so much impact and feedback. I seriously doubt if it is ever available to implement as a neat and good-looking solution - sounds more like an experimental WoG option.

Do you mean scripting reinforcements too?

It's just not the Heroes 2 concept. If you are to implement additional "hidden" slots or something, you will have to redesign all interfaces regarding the army hero, and all that. Not even mentioning that it's not demanded by the community as well as it takes a real lot of effort.

Maybe new summoning spells for heroes or gladiators - instead of just elementals, you could summon different other creatures, that don't persist after battle?

Not that atmospheric and again not too demanded with a lot of work required. Only the soulless entities would be good enough for this - I've answered in the Summoning thread.

By the way, the Reserves idea come from a fan game King's Bounty 2 (1990) by Sergey Prokofiev. Yeah, Russians and copyright cannot be written in one sentence.