Heroes 5.5 WarGame Edition - Tactical Edition.

Are there any plans to make the heroes more diverse in the new duel mode, by adding more abilities or spells?

I don't know which version you're playing, but in the current version (0.99), each dueling hero (as a representative of a class) is unique in terms of stats, as well as in terms of their primary schools and available spells (orcs are unique in their distribution of stun and luck).

Further division of heroes within duels is not planned, because the main point of duels lies in the troops, not the heroes. For full inclusion of heroes in the battle (with constantly changing (rather than predetermined) upgrades and artifacts), there is the RTA (Random Tactical Arena) map mode. This year, we are planning to implement such an Arena mode specifically for the WGE mod.