For a while, the rental market was actually really good when using Splex which automated the entire thing. After the introduction of the listing fee, while reward cards became soulbound, the entire card rental market became worthless though.

Imo what they should do is create new fun game modes with rewards that include prizes like Riftwatcher packs so the only way to get them is to play the new game modes instead of just buying the packs. These game modes should have a fixed fair entry fee for everyone and not require players to own, hold or rent any cards. Instead, cards that are used come from players who just stake theirs in a pool and part of the entry fee gets distributed to them as rental income based on how often they actually are used by players.

This creates a 100% level playing field and no pay2win advantage, just skill and luck and everyone paying the same fee to play. Rental income for players is also really easy, stake once and you earn passively from there on out also attracting passive investors. Part of earnings can go to DAO / Splinterlands Team / Rewards for players / those that provide cards. The really rare and powerful cards also will get a big value boost.