I think the easiest is to take a redemption fee or a trading fee. The redemption fee having the advantage of allowing value capture, even without the Gnosis DAO controlling the marketplaces.
But other approaches could also work.
In the current environment the goal is to attract users to position Gnosis as the main player in the prediction market space. I think that current fees would be negligible compared to future fees, so I don’t think activating them should be a priority (the GNO rewards incentivize the LP and the absence of fees incentivize the traders).
We saw other projects successfully launch liquidity mining program before activating fees (ex: Balancer, Uniswap). Seeing market reactions, I think market participants understand that initially the dapps are to be subventioned, but that once they become large and highly liquid, it’s OK to add fees (the benefit of liquidity outweighing the cost of the fees).
I think that in general AMMs have been the most efficient way to get liquidity (providing liquidity via order book is reserved to professional handling trading bots). But order books should ideally complete AMM. We could also incentivize orderbook liquidity but in this case we’d need to determine how to compute rewards (the good part is that the value of conditional tokens is bound, so we could easily set up a scheme rewarding people making orders in function of how close their orders are to the current price).
For Conditional Tokens, we could have AMM with automatic withdrawal of liquidity at a specific time to protect LP from losses when the event is known.