Expansion of HoMMV? If by "expansion" you mean "alpha-testing new features that might appear in HoMMVI&q
The original game gave you six chapters to slog through, each with 5 missions to complete. This game gives you half the chapters, plus 1.5 new factions. 1.5? How is that possible? Because you get one brand new faction (dwarves), and a second, modified faction (Haven rebels), which I count as a half. You use the Haven rebels for a large portion of the first chapter, and dwarves exclusively in the second chapter. The last chapter is a re-hash of the Dungeon faction (dark elves). Unfortunately, most of the missions don't really show off the abilities of these new factions very well. For example, the Haven rebels don't have any town, so you'll never recruit them from a town, only pick them up from neutral stacks. The rebels themselves are mostly modifications of Haven troops to behave like a different unit type from another faction. The rebel Angels now have the vampire ability to drain life. The rebel footmen attack twice like a minotaur. Although this could translate into a huge increase in Haven tactics if you combined the rebels with regular Haven troops, you never really get an opportunity to do so. Of course, this could be incredibly unfair to the other factions, but creating a spiced up, yet balanced game seems to not be the intent of this expansion. The dwarves are not very well featured either, despite the interesting new abilities they bring to the battle. Aside from some unique unit abilities, probably the biggest addition is rune magic. Basically, whenever your unit's turn is up, you can spend resources to temporarily give them a new ability. The most basic one (requiring only 1 wood), doubles their movement, thus giving your powerful, but slow, magma dragons the chance to run all the way across the field and attack in one turn. This really tilts the scales into your favor, especially against neutral stacks. Another highly over-powered example is the Warlord unit. When attacking, the warlord does a significant amount of damage to not only its target, but additionally does an equal amount of lightning damage to all adjacent targets (including the one you just attacked, so it's like double-damage). It basically equates to a chain-lighting type of attack, but does not have a limit on number of units it can affect, nor does it have any damage reduction for sequential targets. So, if you do 1000 damage to your target, you will do 1000 additional damage to that target, plus 1000 damage to any targets forming an adjacent chain. You could literally do 1000 damage to every stack in the enemy army. Although this is easily avoidable by not making adjacent stacks, the neutrals are not that smart. If you couple this attack with the rune magic that doubles your attack damage, the developers just invented an over-powered tier 5 unit. Of course, actually getting to test out all of these different units is relegated to just a few missions that have a functional dwarf town to begin with.
However, tilting the scales seems to be a running theme throughout the expansion. A new environment type is snow, and for some reason, dwarves like to travel across snow (show me a 4 foot tall person wearing a skirt who can travel faster across snow). Other heroes will be incredibly bogged down, giving the dwarves a huge advantage in travel times. The missions are set up such that this advantage is almost necessary, but how this becomes an interesting add on versus an experiment in movement modifications is beyond me. Unfortunately, some missions require your non-dwarf heroes to cross this terrain, and the game becomes tedious to travel so slowly (which is partially how the game can be considered lengthy).
Another theme in the game is providing seemingly useless additions. New items/characters/monsters seems like a good idea, but in the end, amount to nothing added since you probably don't care anyways. And when that happens in an expansion, the game stinks of lazy development. Even some of the new stat bonuses are somewhat arbitrary and thrown in for the sake of completeness. For example, a warrior guild will increase your offense or defense by 1. Ummm... the old battle arena did the same thing, but did +2. How is adding a +1 building expanding anything? Especially since the warrior guild looks more impressive, graphically, I was dumb-founded that I was getting such a crappy stat bonus.
Despite all of the bad press I'm giving this game, there are TWO cool new additions, and I'm assuming these will be carried through to subsequent expansions/games. First, creature growth now stacks in towns and creature buildings. This means that if you forgot or didn't have the money to buy all the paladins in town, they carry over to the next week, waiting to be purchased. This is awesome because, especially with creature buildings that may be far away from your town and could only be bought when a hero was present, you are no longer penalized for not having the time/money that week to pick them up. Second (and this really ties in with the first one), you can mail-order your creatures and not have to visit any buildings/towns at all. The game calls it caravans, and though not perfectly implemented (you can only caravan creatures you are buying at the moment, not ones you already have stationed in town), it is a huge help in terms of map-management. The down-side is the creatures still have to travel to their destination town, and to get the creatures, you will have to go to that town to pick them up. Hopefully there will be a feature wherein you can have reinforcements sent directly to your hero without requiring another hero to taxi them over. You would think that with this HUGE time saver that more interesting additions could have been made to make the game more interesting. But, you actually waste a lot of time following winding paths on rugged terrain. This is NOT the way to fill up some game-time saved by a good idea.
Overall, this game was a carbon copy of the original (well, duh... it's an expansion), with 2 significantly positive gameplay additions overshadowed by a huge mess of poorly implemented other additions. What this probably means is that we should see in the next expansion or installment at least the caravans and creature carry-over aspects, but the time-saved in this expansion was wasted on gimmicks and alpha-testing new factions.