Comments on my first attempt at a design (if you would)

[JAZZCNC]

Everything the others mention plus a few more.
#1
Widen the Y-axis rail spacing, you have plenty of space so use it. Also, this will allow easier access to the bearings bolts, the way you show it now looks like you the X-axis bearings will block access to the Y-axis bearing bolts.?

#2
The triangle plates at the rear I would move to the outer edges and fasten to the side of the columns, this will give a wider footprint, I'd also think about adding some lateral bracing to add sideways support. Like muzzer mentioned the column risers look a bit flimsy so maybe beef these up.

#3
It's been said but worth mentioning again because it's a Biggy. The Z-axis plate needs to be double the height and the spindle housing needs to be wider.
Also, the spindle motor plate needs supporting or the vibrations will kill the spindle bearings and vibrate on the machine general.

Good luck.