Depends on the type of spin off.
Fallout New Vegas turned out fine (it is pretty much Fallout 3 or as close to Van Buren as we old fans will ever get) so I would love to see more such games.
Tactics? Sure. It had its flaws but I still really liked this game and was willing to retcon it into official Fallout lore (until Van Buren's design documents were shown and I have to say I liked it over Fallout Tactics. Sigh, I want Fallout Van Buren)
Even FOBOS could have worked if it had been better designed and more faithful to Fallout Lore. (No I am not talking about the secret Vault Tec vault, more the ridiculous design of the raiders, the Deathclaws, Harold). Long time ago I had been thinking about how the Shadowgrounds engine could have been used to create a Fallout mod with the player perhaps being a NCR Ranger and having to track down a renegade NCR military unit that is part of a conspiracy involving military take over of the NCR, going to places like the Boneyard, Shady Sands/NCR and some new locations such as Vandenberg AFB, Diablo Canyon Nuclear Power Plant, and Area 51, and meeting factions like the Followers of the Apocalypse, the remnants of the Unity, and the Brotherhood of Steel.