Adding Monero wallet support to Bisq would be a feasible first step and will be still a valuable feature if we move one day to the off-chain trade protocol (https://github.com/bisq-network/proposals/issues/32).
Adding XMR as base currency is a huge effort. I would estimate 3-6 months for a dev experienced with Bisq and Monero.
I think we should focus instead on the off chain trade protocol where there is no base currency anymore.
The currently used multisig based protocol has several issues (which would be only partially solved if Monero would be base currency):
- Miner fees are high (4 tx are required, maybe 2 can be omitted if we find a optimization but not clear if possible). Once miner fees are high again that will be a burden. Not sure about XMR miner fee future outlook but it can be assumed that it will be a problem at some point as well
- On chain privacy issue with connected txs. As Bitcoin on chain privacy is pretty bad that is an issue. Not on Monero.
- Arbitrator as 3rd key holder carries unclear legal risks and has some security and centralisation issues. -> We are preparing a proposal to remove arbitrators completely and use a 2of2 MS instead.
- Trade speed depends on base currency and counter currency transfer. At BTC its 10 min. block confirmation. Depending on the altcoin side it adds another few minutes. With Monero it can be faster, but also has the limitation when doing repeated trades that Monero cannot spend own unconfirmed coins, so delay will be even longer.
So you see the tx fee issue would still be a problem and more important the confirmation time will be still a limiting factor for trade speed.
With the off chain trade protocol the XMR transfer will still be a part but at least it does not depend on the trade protocol. I users find an alternative transfer channel (like lightning network in BTC) it can be elimitated or at least optimized. At least on a XMR-BTC trade you could use lightning on the BTC side and have only the confirmation time on XMR as limiting factor.