I don't own one, but I usually see lots of raccomandations for the EdgeRouter Lite or the EdgeRouter POE (which is a Lite + integrated Switch + POE).

It does run on Debian, I think you can just ssh in an do everything you want. By the way, on DD-WRT/OpenWRT/Tomato you can install pretty much what you want by using OptWare/EntWare.

The problem is usually not the software, but the hardware. Router are still slow (for example, I tried once to use transmission on my puny e3200, and the speed was capped at ~350kbps by the CPU) and they don't have lots of connectivity (on high-end routers you get maybe 2 USB 3.0 ports. The WRT1900AC is in fact the first router with an eSata port, and the first one with decent speed over USB).

If you want to run anything somewhat "heavy", you're far better with having a different box, or using a custom pfsense/smoothwall/m0n0wall/vyatta box.