The community will be able to vote which coins can pay tx fees in their own token…and (i believe) stakepools will also be able to choose if they support these tokins too (incase pot coin is illegal in your jurisdiction). The pool deligators will also recieve these tx tokens, which will create a interesting marketplace with stakepools.
In the early days though its better to be just ADA. Imagine if you open your wallet to find you have been paid 500 “other” coins last epoch. That would be great if it was "maker, aave or uni. But not so much if it was verge bitcoin diamond or tit coin.
I imagine someone could create a automatic uniswap like application dex that makes this seem seemless from the token senders point of view and pays the fees in ADA. (Any devs here wanna tackle that one?)