I'd vote either DiabloRL or BerserkRL, both of them are good. But to make a proper DiabloRL you'd need to invest a lot of time in it, probably more than you ever did in DoomRL. To add six classes, skills, a lot of spells, massive number of quests, huge amounts of magical items and enemies plus the amount of playtesting needed to make everything work means this is a really big project and god knows how long it'd take to finish. Definitely not suited for a "playground".
On the other hand, BerserkRL is a rather simple game, since it's basically an arena where you fight nonstop. All the equipment is already there, so work is only needed on the skills, enemies and some other details. Much easier to work with and shouldn't take long to complete.
So my vote goes to BerserkRL.
