One person’s perspective but here’s what I’d offer as evidence a new difficulty level is needed on solo raids:
(1) I’ve only been playing for approx 8 weeks and I figure for at least the last 2, if not 3, weeks I’ve been able to do all three solo raids on brutal despite being just a ‘mid-level’ player (lvl 61, VIP1) and being far under the recommended power level (even now I believe I’m still prompted about being low power for one or two of the raids despite being able to auto them w no issues). That said, another solution to this particular issue may be to make the existing raid levels harder but there are probably reasons you might not want to change them.
(2) I’ve started bringing one hero from each colour/type to platinum. The reason I’m doing one of each of the three colours is because of the scarcity of cannisters. It’s kind of crummy being dependant on, in my case, 45 bronze cannisters to complete each upgrade and knowing that means waiting approx a week to get a bar (5 cannisters for brutal x 3 raids x 3 days a week = 45)