I've been playing through the main campaign. I'm only on mission 4 and I'm having an ub-believable blast. I'm still very new to the series though (just scored all of them this past weekend with the sale :D ), so excuse my ignorance.

In my many hours already clocked playing, I've noticed heroes disappear and de-level after every mission in the campaign, while Isabel maintains her level and reaches the level cap for that specific mission. She's in her 20's already, but does any other hero ever get the chance to get to high level and stay there for the duration of the campaign, or does every hired hero disappear after every mission, thus giving the need to be hired and leveled again?

None of the heroes you hire yourself get carried over between scenarios. Only unique heroes that you're given automatically get carried through.

All the heroes you can recruit in taverns are from a pool of heroes that have their skills reset for each map.

In some campaigns you can carry other heroes forward to the next scenario. The scenario description USUALLY tells you how many and what their level cap will be.

This is what I was wondering! Thanks!

It's too bad they don't carry over for every game though :(

Is there any strategy game that carry's your army over to every map or campaign?

I'd say King's Bounty would fit that criteria best. I can't think of any other strategy game that allows that off the top of my head.

Carrying over one hero is already a big boon to any army you recruit under them in your future campaign though. I assume you are playing HoMM5. Every campaign has a hero that let's you carry over his/her level to the next one. So make sure it is your primary hero to use in most fights.

http://www.heroesofmightandmagic.com/heroes5/knight_skills_abilities.shtml
You can also use this link to plan your future skills for said hero. I personally enjoy using that because I'm a tad OCD when it comes to picking the right things at a level up, although keep in mind that some people consider this a "spoiler." If you rather discover everything on your own then you can freely ignore it.

If all your heroes and/or armies carry over, your faction would be incredibly powerful and makes the next level too easy or imbalanced. Sometimes the game recognises this and penalise you by making you start with less resources, to make up for the fact you have a strong hero.

In Heroes 3, the "Spoils of War" campaign allows you to take all heroes (with XP and skills intact) in the final struggle. There are probably other examples from H4 and other games in series. I can think of a few games, but they're all historical and not fantasy.
'Close Combat' series being one.
All dead/injured men and equipment reflected, so do the map damage.

I am indeed playing HoMM5. I tried playing the first couple games only to find them very hard. I enjoy hard, but a first play through it sort of slows down my enjoyment. Five is considerably easier to get into, but from what I've read they tweaked a couple things to serve this purpose and to standardize the series with other games around it's release.

I'm really enjoying 5, but have every intention to return to 1 through 4, regardless of difficulty. I don't mind historical games. It need not be fantasy.

Speaking of historical strategy, I regret not scoring the Panzer General's and the Imperialism titles this past weekend. But I might still dish out the cash for the full prices of them all. I need to have more historical diversity in my strategy collection and not just sci fi and fantasy... Makes me feel more complete! And I love history.

Fantasy General (and the 5 star series in general)
Your entire army earns XP (unit by unit) and you move all of them from one battle to another. The campaign is split into 5 continents (if I recall correctly) and there are some units that you recruit on continent that stay on that continent when the rest of the army moves on.

You can name the units - which makes it HURT when a tac-air squadron (or giant phoenix like birds) gets killed off FIFTY battles into the campaign! "they killed 'silver-bells'. Those bastards!"

Somebody mentioned King's Bounty. The Elven Legacy games are sort of like that too. Ditto Fantasy Wars (which is basically the same game as Elven Legacy with differnent maps. Even the tutorial is the same)

Fantasy General got be snatched up at abandonware sites.

Fantasy General sounds amazing! If only it was on GOG :(. I've been so GOG crazed the past number of months it's the only place I've purchased PC games. In the meantime I'll peak at Kings Bounty soon and probably aim to get Panzer General's and Imperialism 1 + 2...man I should've scored those when they came out on sale!

Since you have the whole series, you might want to play III or Chronicles. III is more varied and more difficult, Chronicles is easy mode. Armies and artifacts don't transfer, but both games have more hero carryover than V: typically, the campaign hero + two highest-level hired heroes. These hired heroes can be whoever you want, but, unlike the campaign hero, they can lose battles and be normally rehired at taverns.

Celestial Heavens has a much better skill build wheel that lets you choose the Hero you are using and it's easier to see what you need all on the same page without having to scroll much at all. It also gives you a level count so you know what level you will need to be before you can get there.

Example: The Demon Lord Hero Deleb, the one with the Fire Ball Ballista, can get the ultimate skill by level 25, or the Tripple Ballista at level 4.

http://www.celestialheavens.com/viewpage.php?id=520