Beware! long post ahoy!
Well firstly, I'm very pleased with this idea, as probably everyone will know, I've been wanting an Audio Rpg for ages!
As reguards how the game works, Imho a mixture of styles would be best for the game. Having only one style ---- say 2d side scrolling action, would limit the game to being basically a glorified version of Superliam, where as doing a last crusade style would limit the game in other ways as you noticed yourself.
One alternative might be to have the game working through menues. So, how about starting with a menue, asking if you want to go to the village of waterdeep or the town of castlerock (please excuse unoriginal names). You select to go to the town, and then must go through a short side scrolling section in order to get there in which you fight monsters.
Once in the town you could have various locations such as shops, or Npc's to talk to on the menue.
You could also do dungeons on this system by having each dungeon room separated by a short side scrolling combat. So you select "Explore the eastern corridore" and have to fight monsters before you get to the room at the end of the corridor. Imho this would probably be the simplest to create.
Another map alternative could be a grid system as seen in the Vip gameszone games, with a constant movement throughout the squares and a non-scrolling combat occurring whenever you found a monster. So, you move north from square 1.1 to 1.2 and encounter a monster, a voice tells you the type of monster and instantly you need to block their attacks with your sword from left to right, as well as using arrows or spells with other keys.
this would make the game far more free and give a lot more exploration, but imho would be more difficult to make, especially sinse dungeons would have to work in a similar fashion to mortalmaze.
Of course, using either map system with a turn based combat is possible, but it would probably take a lot of work to make the combat system sufficiently complex to make it interesting (the one in Last crusade certainly wasn't! complex or interesting enough).
Obviously which ever mode you adopt there needs to be a good ongoing plot, Npc's to talk to, weapons, armour and other items to acquire and use, and some system of leveling up and ganing experience.
I'm just wondering, why you feel it necessary to develope charactor classes? There are many different systems you might considder, iether having one charactor who acquires skills and magics throughout the game (the way Link does in the Zelda series), or having a party composed of two or three charactors, one of whome is a warrior, another a mage etc.
of course, it will all depend upon the plot and setting of the game, I'm just presenting a couple of alternatives.
Btw Robjoy, I'd be very pleased to help in any audio Rpg developement in any way I could, so please feel free to contact me.
With our dreaming and singing, Ceaseless and sorrowless we! The glory about us clinging Of the glorious futures we see,
Our souls with high music ringing; O men! It must ever be
That we dwell in our dreaming and singing, A little apart from ye. (Arthur O'Shaughnessy 1873.)