OP, as someone who upgraded from the Sundara to the Arya, only to return the latter, I'd say there's no need to upgrade at all! Yes there are headphones that have better technicalities out there, but over time a lot of us find that tonality is more important - and the Sundara has a great tonality imo.

Higher end cans are still lots of fun to try, so I'm not discouraging you from trying them, but if you're already enjoying your music tons with the Sundara, well isnt that essentially what people are looking for in upgrades? And you've already got it.

I'd say Sennheisers are great complementary cans to the Sundara; I'd recommend the 6xx or 650 (whichever is cheaper or looks better to you, sound wise they are essentially the same), or a 660S. Having variation keeps things exciting.