What if MS, when BC is fully implemented this fall, announced in march that all of their 1st party 360 games released in the BC program would be included with the purchase of every new Xbox One? How big of a selling point would it be?

It will be like seeing with your open eyes a sales gap, to then close and open them again to see it vanished.

Or at most, it would help Xbone as much as a starting PS3 got helped in sales by remaining compatible with the best selling console of all time.

But imagine: "Xbox One - starting at$399, now with 30 of the greatest games ever included" or something like that :P.

Would be great with a price cut

Sounds like a pretty good deal, but I'm not sure there are too many people out there for whom this would be a great deal, other than traditional xbox fans.

It's my impression that the lead 360 garnered was due primarily to four factors.
1. Out a year earlier, leading to larger sales gap spiral
2. Cheaper
3. Momentum AAA brand entries
4. Much better performance in multiplat games

Currently, xb1 only has #2
I don't think Halo and Gears (gears now owned by ms) are as popular as they once were. I think the steam is running out on them, on top of other factors like dev changes, and increasing lower critical success over entries. People who want the old ones are going to be old fans. New purchasers are going to want the new, exciting games. 

In summation, I don't think people in the market for a new console are going to be persuaded by deals like that. Because while they would be quite a numerable bundle (6 forza games, 6 halo games, 3 gears games, 3 dance central games, 2 viva pinata games, banjo nuts and bolts, 3 fable games, a few kinect games, 2 crackdown), it's also not a very varied bundle, and stretched a little thin. You don't want to give people too many options because then it gets a very "as seen on tv" vibe. You know, like, you bought the game you wanted to buy, not the game that said 30 games in one, because you knew you weren't going to play 90% of those, and it didn't include the one you REALLY wanted to play. Those 30 in 1 type deals were the ones your parents bought because "a game is a game, right?".

Honestly, the best deal to me would be:
Halo 4, Halo Wars, Gears 3, Forza horizon, minecraft, fable 3
and that's it.
Maybe a voucher for 10$ off Halo 5 digital copy as well.
That ensures that your offerings feel recent, concise, touches every major genre, and doesn't oversaturate.

EDIT: I didn't read your recent post that says "includes 30 games". I thought of that independently. 30 in 1 is not always a good selling point.

Yep all the work of makeing older gen games work, not through software emulation, but basically porting every game ever from the 360 to the One, and then giveing all that work away for free.

Ontop of that, if you buy a One and it comes with like a bazillion free games, then that means people buying the One will probably buy less games than if you just sold them a console without tons of games for  it.

This would be a reeeeeeeally expensive "idea".

I know MS has tons of money, but I bet they want to earn abit off of their consoles as well... otherwise whats the point?