I've noticed a trend that pops up rarely on Steam. It's when two Steam developers decide offer an exclusive, free piece of DLC for their game in exchange for owning another game on Steam, which is automatically handled in the background by Steam.
We've seen this with the following games, almost exclusive indie developers.
Terraria + Edge of Space: Terraria owners get an EoS armor, Edge of Space owners get a Terraria armor.
Battle Block Theater + Castle Crashers: BBT owners gets a Castle Crashers character, CC owners get a BBT character.
Awesomenauts + Sword and Soldiers: Awesomenauts owners get a free skin for Voltar styled after an SoS character, I forget what the other side receives.
People who enjoy one game have a tendency of looking into another game if it offers something small they may want in their original game, and see if the game appeals to their interests enough to want the cross-promotional content.
It seems to me that indie developers only benefit from this sort of thing and might take into account swapping player interest to broaden their impact. Several examples I can think of might be along the lines of...
Defender's Quest + Dust an Elysian Tail: Defender's Quest owners receive 'Dusty Book', which fires a slow stream of tiny projectiles similar to Fidget's from Dust for 1 damage to any enemy immediately near Azra. Dust owners receive an extra NPC in their cottage for free, Azra or Slak, giving an HP bonus the same as all the other NPCs do.
Defender's Quest + Epic Battle Fantasy 4: DQ owners receive 'Epic Book of Swords', giving all berserkers +% crit chance. EBF4 owners receive a special unbound 'Psi Storm' skill that shocks random enemies between # and # times.
Defender's Quest + Secrets of Grindea: DQ owners receive 'Book of Collecting', giving +% to scrap gain. SoG owners receive a visual-only Obsidian Skull hat.
Does this come off as a terrible idea, or is incentivizing networking between indie developers or strengthening existing connections ideal and a smart path for the future? Any other possible venue of promotion than just relying on curators, youtubers and press can only be a good thing in my opinion.