My Submission (updated):Neutral Ground • $4 • Action - Victory - Liaison
+1 Action
+$1
+1 Favor
Worth 2% if you have at least 5 Favors (otherwise worth 0%).
Made a new version. I originally wanted it to be a dead Victory card as a strong contrast to all of the existing Liaisons, but it subsequently occurred to me that that would render Circle of Witches and League of Shopkeepers useless (in the absence of another Liaison); and, since (without the Favors) the card is a $4 Estate, it would (mostly) render Neutral Ground useless. While a 2/23 chance of the card being unusable is probably not prohibitively bad (I would posit that there are several official cards that are mostly unbuyable in a larger portion of Kingdoms), I feel like it isn't in the spirit of the mechanic to have it just not work with some Allies.
Thus, I came up with a new version, an Action - Victory - Liaison. I like this new version better anyway. It's a disappearing Copper that also gives a Favor (or a disappearing Bauble with only 2 of the choices), and if you save 5 Favors it's also worth 2VP. There are some fun synergies with Fellowship of Scribes (which makes it into a [conditional] Peddler, but if you want the VP you'll have to forego 4 draws) and League of Shopkeepers (which, after 4 plays turns it into a much more valuable disappearing Silver that is worth VP).
(I also forgot to give my first version the Liaison type. Oops.)
Old version:Neutral Ground • $4 • Victory
1VP
When you gain this, +1 Favor per Neutral Ground in the Supply.