They can do RDM easily, they just need to find a gameplay for it and put melee attacks and spells animations and here goes your RDM. You could even take an already existing class as an example : DRG

2 Combo, the debuff combo and the DMG combo, 3 non buff/dots :
Makes the Debuff combo have 2 fence skills and finish it with an instant thunder spell for dots, makes the DMG combo the same, 2 sword skill that ends with an instant spell, have the non combo buff/dots be instant spells or buffs and you have your RDM.

You have to think out of the box. Don't think of it as a jack of all trade, but as a job that can use melee skills and magic skills and you can open new possibilities.