Really random take here, but one thing I love a lot about modern Doctor Who is that we're beginning to see a lot more of the classic designs of some of the monsters. 60s-era Daleks have appeared a few times, there was the special featuring all of the Cybermen looks (with the Tenth Planet one as a main character which I actually love) and then more recently, the Fourth Doctor-era Sontarans in Flux. I personally love seeing this kind of thing - especially when you consider the nature of Doctor Who as a series about time travel, visiting civilisations and species at different points in time would be quite interesting (and, generally speaking... is kind of a missed opportunity overall.)
It's also why I like it when monsters from the older series, like the Autons or the Zygons, return in a genuinely interesting and well-written way (unlike the treatment of, say, the Weeping Angels.) I think the Autons especially were very well-done back in Rose, really delving into the potential of a situation like that in the modern era - treatment that a lot of other monsters, such as the Wire as seen in The Idiot's Lantern, would be fun to have (I know it's more modern but it's SUCH A COOL CONCEPT.)
For instance, imagine a modern story featuring the Ood but before they were enslaved in the 39th Century, having their connection to the Ood Brain explored and their connection to the Doctor observed (as well as, perhaps, their relation to their cousin species in the Sensorites!) or an ongoing story similar in concept to Flux or Trial of a Time Lord with a main villain in the form of Omega - or even just to see older villains like the Axons, the return of the Wire in a modern context, or, in an ideal world... the Chumblies. (My beloved.)
