You should put soakers on your blades after you dry them off - that alone will prevent any scratches. If you really want to use the suitcase, repurpose some old hand towels. For each one: fold in half to form a pouch and see if your skate can fit inside. If it can, stitch or pin the sides to form a pouch. Put each skates inside a pouch then put them in the bag and they won't touch each other. If you have leftover fabric in your craft supplies, you can use skate measurements to make the pouches, adding a few inches around to account for the skate width and the blade.
I have used a rolling backpack for my skates for many years. I've tried a suitcase and didn't like laying it down in the dirty floor to get at the skates. The rolling backpacks stand up by themselves, so you can get at everything. As you pointed out, everything shifts south when you stand it up so I was always digging for my things. When I used a suitcase, I attached a mesh pencil case to the carry strap to hold my skate rag so it could dry. I also left the zipper undone at the top, to let air circulate. I put the skates in with the toes in opposite directions and the laces facing/touching. Then, I piled my notebooks and papers on top to hold them in place since the straps were useless. You *could* add some mesh fabric to hold the skates, if you have any.
Tried a regular backpack but we don't have lockers, so I have tons of crap in my skate bag and it's too heavy as a regular backpack. In a pinch, I can use the straps to carry the rolling rig over areas where you can't roll easily. (Mud, cobblestones, unpaved areas, deep snow.)
There's a season for rolling backpacks that starts around this time and ends around October. Apparently, it's considered a "back to school/off to college" item for brick-and-mortar retail. You can buy them online but the prices are higher during the "off-season." The best one I ever had was from BJs ($30?) and it was perfect but wore out after 5-6 years.
I put some crap on the bottom - as Agnes says, leftover insoles, rolls o' tape, old soakers to be given away, etc. I put the skates in first on the backstraps, then put my skate rag and screwdriver in one outside pocket, my socks in the other and my spinner/guards in the strap pocket on the backside. There's usually a front pouch where I keep my skate repair kit (screws, toothpicks, small awl, extra laces) and other crap (bandaids, business card wallet, notebook, pen/pencil, markers.) My skate hook is on a lanyard, so I loop it around the top carrying handle and tuck the hook/strap inside the bag so it's easy to find.
That "best ever" backpack also had a top compartment for a CD player (long time ago) with a port to push a headphone through. I used that compartment for all the little things that get lost and some spare change for lockers. Really handy, but not mandatory.
I NEVER close my bag or any of the pockets all the way, so ventilation isn't an issue.