The Godol's post on Yom Hashoa makes some good points. There are the pros - that The Holocaust definitely deserves a day of remembrance because of its magnitude, and the cons - non-traditional, in Nissan, commemorates particularly the Warsaw Ghetto uprising, which is what resonated more w/ the Zionist founders of the State of Israel (cuz' they didn't go 'like sheep'), etc.
Here's my proposal; the 20th day of Sivan should be commemorated as Yom Ha-Sho'ah.
The 2oth of Sivan was originally declared a fast day in Ashkenaz in the 12th century, after a town was massacred (I can't remember the details, but it's actually quoted in one of the major commentaries on the Shulchan Arukh). It was observed as a fast day for centuries, but gradually fell out of observance. It was renewed in the 17th century as a commemoration of the Chmielnicki massacres, and no less than the Shach, R' Shabsi Cohen, penned a piyut to be recited on that day. It only fell out of general awareness this century. Older and more complete siddurim still have the slichos for the 20th of Sivan.
Thus, it's a day that can't be considered non-traditional, it's not on Tisha B'av, it has a basic liturgy in place, and can be a commemoration of the Holocaust without forgetting about the other great post-exilic European tragedies (the expulsion in 1492 happened on Tisha B'av, making it easier to include there).
I think it's a darn shame that this date wasn't originally selected as Yom Ha-Shoah. Is it too late?