The title says it all. It's a crazy idea. I'm not saying there SHOULD be neutral legends, or that there is a definitve need for them. I just like to tinker with the idea. So having said that, how could one make it work?
I figured one would begin by adressing the issue of cost first. Seeing as neutral cards don't provide loyalty (and legends usually requiring 4 or 5), having them cost more than "regular" legends seems fair (unless you want to dillute their potency, thereby making the idea redundant in itself). But having legends costing 8-10 resources seems a bit steep, doesn't it?
So how about if they had a cost reducing effect depending on the amount of "associated" units and supports in play under your control? I.e. if you have say, 4 Undead units or supports in play under your control the associated legend - for instance, let's say it was Nagash - would cost 4 less (with a minimum cap of maybe 4 or 5 resources, so as not to make it silly cheap).
That way you have a way of reducing the cost of playing neutral legends, a kin to how loyalty is required to play a non-neutral legend. Although ,it might be better if this was some form of additional cost (e.g. cost X more resources to play unless…) , and not directly tied to printed cost (to avoid shenanigans).
The second issue is of course, what makes them worth all the trouble? They would only be reasonable in decks with a lot of neutral cards of a specific neutral faction. So maybe they should share some of that factions unique traits, or a more potent version of it? Let's look back at my proposed Nagash, shall we, and think about how it would interact with a Undead themed deck.
Necromancy comes to mind (obviously), but what's good about bringing in a legend for just a turn? Not to mention the keyword only applies to units. As it would have to be card text anyway, why not state that the legend in question can be played from the discard pile as if it were in your hand, and leave it at that? Works wonders with the Undead theme of self-milling.
Then we have the need for some form of action or constant effect that really makes this legend shine and enforces its character. Going back to Nagash, who was a great necromancer and mage, it's not far fetched to give him the ability to play spells from your discard pile (maybe returning the to the bottom of the deck after they have resolved). Maybe limit it once per turn.
Finishing touches with power and hitpoint distibution (I'd go with 2 and 4, respectively), and voilá - a Neutral legend is born (or reborn, or unborn, or whatever is appropriate in this case)!
Nagash (Neutral Legend, Undead) Printed Cost 6
Power 2 Hit Points 4
You must pay an additional 4 resources to play this card unless you control at least 4 cards with the Undead trait. You can play this card from your discard pile as if it was in your hand.
You can play Spell cards from your discard pile, paying all costs (limit once per turn). If you do, after the Spell card leaves play, put it at the bottom of your deck.
Well. I'm eager to hear your comments and ideas for other neutral legends. Again, this is just a pet peeve of mine and nothing serious. But one can dream, i guess…?
TL;DR - Crazy idea for neutral legends, thoughts? Just for fun!