But what about if I gain a BV, from that gain a Cultist, and then trash the Cultist with Watchtower, drawing three cards? Where would the BV be as I'm drawing cards? I can still topdeck it, so it can't be in my discard pile yet (since Watchtower now redirects gains). So it's still in "nowhere" land. Not exactly intuitive.
Also, Inn would not be able to shuffle itself into your deck, since we would do all on-gain effect before the card is actually gained.
So if i understand you, this means that i can buy BV, gain Cultist, trash it, draw 3 cards and then topdeck BV if, and only if, i didn't have to shuffle inbetween? I see that in such cases there is a difference between my idea for a ruling and the official one, which i didn't intend. Inn is a good point, too.
Fair's fair, i accept it's not working my way.
Here the infamous lose-track rule roars its ugly head.
This is how it happens:
1. You gain BV, it goes to your discard pile
2. You can choose to resolve BV's gain or WT's reaction, you pick BV's gain
3. You gain Cultist, it goes to your discard pile, BV is now covered and lost track of (by WT)
4. You trash Cultist with WT, draw 3 cards (may have to shuffle the BV into a new draw deck)
Whether you had to shuffle or not isn't important. If you didn't, you still can't topdeck BV after gainig Cultist first, because it's been covered by another card and thus lost track of, that's the rule. If we diverge on step 2:
2. You choose to resolve WT's reaction and topdeck BV
3. You then gain Cultist, it goes to your discard
4. You trash it with WT and draw 3 cards, one of which is the topdecked BV
The result of this is that WT doesn't redirect gains in mid-air, it moves cards to another location from a certain location. If the card to be moved isn't in the expected location anymore (top of your discard pile, hand, or top of your deck) or it has been covered in between (meaning that the top card isn't necessarily the one you gained anymore) the WT loses track of it and can't move it.
You could make an argument for covered cards that become uncovered: "hey, I can see the card, it's right on top, why can't I move it now?" but that would make the ruling just more incoherent. This way it may be more awkward, but covers edge cases better.
Simple edge case example: Gain BV, gain Inn, shuffle BV into your deck (not Inn), I believe you can still trash Inn (which is still on top of your discard pile where WT expects it to be) with WT, but it's impossible to move BV with WT.