This is something I could get behind, a bit of flexibility in detachment sizes.
Have infantry detachments start at 4 models, with upgrade options to go to 6. Vehicle detachments could start at 3 and go to 5 etc.
Random question, is such a thing interesting for other factions? Like a Guard one for instance? Just wondering...
I think the appeal is going to highest for new players who don’t feel comfortable moving beyond the existing lists – especially those who’ve started buying things here and there and don’t have the right numbers for the usual formations. If that’s who you’re targeting, then a kitchen sink list would throw out “X support formations per company” limits and give broader ranges like 2-6 Predators @65 pts each (it doesn’t have to add up to the exact values in the main lists). And because of the support formation limits, it’d be even more valuable for non-Astartes armies.
We’d probably want to encourage players using these lists to play missions from the various supplements people here have written. Get creative when taking a non-standard army instead of playing the same ol’ Tournament Scenario with potentially unbalanced lists.