The first few that occur to me:
Furtwangler in the symphonies.
Oscar Fried in decent sound.
Mitropoulos in the 7th.
Ernest Borsamsky in the rest after his rather good no.1
I thoroughly enjoyed Mark Gorenstein in the 5th [he conducts the Russian State]; so him in the rest.
Toscanini and cantelli in any/all.
And Mravinsky and Silvestri.
Horenstein in 2 and 5; both exist but haven't made it to commercial recordings yet.
Walter and Klemperer when middle-aged.
And for real fantasy stuff, Mahler himself conducting orchestras.
Ivor
Furtwangler likely knew about the symphonies, but for career (and likely racial as well) reasons, never had any desire to conduct them.
It would have been good if Fried had waited until electrical recording came about to delay his recording of M2. However, with his death in 1940 or 41, we wouldn't have gotten any Mahler recording by him in decent sound, considering he emigrated to the Soviet Union.
M7 is one I believe I quoted earlier that Mitropoulos should have recorded.
Can't comment on Borsamsky, Silvestri or Gorenstein, though Silvestri might have been interesting.
Toscanini and Mahler disliked each other intensely when the former came to New York about 1908 (I think) as a Metropolitan Opera conductor; it only portends that he should have disliked Mahler's compositional output as well. With Toscanini later in life taking on Cantelli as protege, he would certainly have communicated his dislike for Mahler's output to Cantelli; therefore no Mahler recordings (much less performances) by either man.
Can't comment on Mravinsky, and don't know if there is any documentation on his like/dislike of Mahler. Maybe Mravinsky's student, Yuri Temirkanov, could say whether or not Mravinsky liked/disliked Mahler. Temirkanov certainly does, as I've heard him conduct M2 live.
Someday, we'll have M2 and M5 by Horenstein (likely after our lifetimes?).
The 1938 M9 by Walter was conducted by Walter when he was middle-aged. About the only significant Mahler document by him from that period. No one wanted to perform Mahler during those times for career, racial, political, aesthetic, and economic reasons.
I think Mahler would have liked to record his works once the technology had reached a decent point where most every instrument in the orchestra could have been heard (i.e., electrical, no acoustic recordings, and no instrumental substitutions or deletions). Those piano rolls he recorded are likely his preference as to how he wanted those particular pieces to go (at least from tempi and dynamic viewpoints).
Wade