Sounds like a variant of "if you promise not to bring your Wraithknight, I promise not to bring my Stormsurge."
If you have regular opponents, restricting them from bringing units that you regularly struggle against and/or seem so ingrained in that player's list building you're not sure what he'd do without that unit are a great way to add variety to your games. It relies on both players having sufficiently sized collections to be able to cope with the lost units.
I'd suggest Troops choices being exempt - most armies have enough
HQ options that forcing a new
HQ can be interesting, but Troops or basic transports are pretty limited and intrinsic. I'd class Drop Pods as an exception to this exception though
The maelstrom idea is interesting, though I'd make it 20 rather than 15 - I can plow through that many in a 7 turn game simply by being agressive with ditching difficult ones and being lucky with low-hanging fruit.