From BostonUrbex:

I give you… the Red Line Wye:

View Red Line Wye in a larger map

Continue the Blue Line connection at MGH under the Charles to a junction somewhere near Kendall. Put a junction north of Broadway, South of Maverick, and connect them. And there you have it, my Red Line Wye, also featuring a new stop for the Financial District and Downtown Waterfront located below the RKG between International Place and Rowes Wharf. Will also feature a free ferry transfer.

Oh, and the Maverick-Broadway portion will run to Ashmont only, not Braintree. And that would cover all Ashmont service. This is an attempt to keep Braintree riders from getting pissy like they do about everything.

And the Green Line Wye:

View Green Line Wye in a larger map

Except I rename her… The Blue Line Wye. The Green Line would be upgraded to handle current Blue Line rolling stock. The curve from North Station to the viaduct portal would have to be straightened out, Haymarket would be eliminated due to new alignment to account for the curve coming in from State, and that would be covered by a new Government Center station on the north side of the wye. Also, Boylston would have to be relocated, if not eliminated, due to curve.

To which I responded:

This is very similar to an early proposal for the original Tremont St subway and the East Boston tunnel. The wacky original layout of Scollay and Adams Sq stations was to act as a kind of interchange for trains coming from the north, east (via the East Boston tunnel), south, and west (via Cambridge St). I think once the subway became popular there was a push towards full heavy rail which killed the idea.

Here is a map I found once, year unknown.

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Mayor Menino’s Crohn’s proposal for a Lowell Commuter Rail Extension into Nashua, NH.

View Lowell Commuter Rail Extension in a larger map

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Finally, omaja’s new system:

I like the ideas of connecting more lines together but I think it might be better accomplished via completely separated, dedicated lines.

Here’s my crazy idea that kind of rethinks the entire way the T is organized now, based on Madrid’s Metro.

A rough sketch, but you get the idea.

View Boston MBTA T in a larger map