P-G: The Marshall is a strong piece. Same piece as in Embassy Chess. I don't think of it as a clumsy piece. In Embassy Chess there's less room to maneuver, especially at the beginning of a game. The Marshall has to be developed carefully to avoid being harrassed by the minor pieces. The Cardinal is able to move diagonally. This gives it added mobility at the beginning of a game with all the Pawns mostly on the board. Grand Chess has lots of room behind the initial set up. With the Rooks being able to get right into the game and also, and this is often overlooked, the King has extra room to run to. This greatly reduces the tension of the game as compared to Embassy Chess where the King can be readily attacked and has no place to go. Sacrifices are always something to be on the look for. Defending against them, or initiating one of your own. Later in the game is when the Marshall shines. Crossing the board and getting in close. Very hard to defend against the Marshall, especially if you have a Bishop or other diagonal moving piece backing him up. Both the Marshall and the Cardinal are hard to stop when close to the action. The Knight movement part of them can't be blocked, so they essentually have twelve direct places to attack when up close as compared to the Queen's eight. Look at a 5 × 5 square on the board and you'll see the Queen, Marshall, and Cardinal all cover the same number of squares. Widen the field and the Queen covers more squares.

As for rules changes to Grand Chess. I think one I'd institute would be moving the Pawn promotion row to the ninth row, instead of the present eighth. Then they'd have to cross the same amount of squares as they do in regular Chess. The restricted promotion of which piece can be chosen is a different rule and seems OK by me, but it does take some planning if you happen to have a Pawn in position to promote when there haven't been any piece captures or just a few minor pieces.

I don't know about these Chinese Chess Cannons. Why not add a couple more Rooks if you like power on the board? Or a Marshall and another Cardinal? Yeah, throw in a Queen too. MIght as well have two of every piece except a King. The back row would also have the room to add the pieces used in Ultima. If you want excitement in your Grand Chess, that'd most certainly do it. An Immobilizer working with a Cardinal? Oh yeah. :) The Longleapers of Ultima are similar to Cannons I believe. The Coordinator is too. The Withdrawer is weak, so make it another Queen. And just imagine the Chameleon being able to attack with so many different pieces that they could copy their moves. Ultragrand Chess.

We used to play Ultimamen versus Chessmen. That's an interesting variant of both games and allowing the Chameleon to move as a Knight against a Knight helps the Ultima side as it seems the Chessmen are very strong with having the form of capture that they have. Seeing how there's a few non-equal sided games on this site, it'd be cool to have this variant too, though I suppose we should first get regular Ultima added too.

Walter Montego:
Thank you, I have to digest your rich and long answer. I will do it tomorrow, because it is a bit late now.

About the chinese cannons, you will find how interesting they are in the rules of chinese chess under "New game".

Walter Montego: It is worth noting that inventor Freeling believes the Marshal is the strongest piece. Many disagree, citing the Queen's better mobility. (Placed on any of the four center squares of an open board, a Queen covers 35 squares, as opposed to the Marshal's 26.) And there are situations where a Cardinal is preferable to either. However, it is widely recognized that Q, M, & C are the strongest pieces. Many newcomers have difficulty grasping the Cardinal's superiority to a Rook.

Walter Montego:
The Marshall is a powerful piece, I agree.
We also agree that its use occurs later in the game.
If we compare the dynamism of the two Janus with the Marshall and Cardinal, we can accept that the start of a Janus game is much more fun.

As regards the Chinese Cannons (by the way have you already had time to read the rules of Chinese Chess?).

My intention is not to accumulate powerful pieces like with Rooks but to add two very interesting and exciting pieces full of new combinations possibilities.

I see no reason why Chinese Cannons are not yet used outside the Chinese Chess. Probably simply because it is unknown is a reasonable explanation.

Actually, all occidental pieces are formed from an association of Bishop, Rook and Knight movements. Why not extend that with a so tricky piece?
I am certain it will give a new breath to our play.

I ignore the Ultima game. Where, please, can I read something about?

Walter Montego:
About Pawn promotion, I have another suggestion than yours:
A Pawn is promoted to a Gold General (piece borrowed from Shogi, the Japanese Chess).
When captured it returns to a Pawn.
No doubt, this will boost the game, due to this terrible threat.

I have not yet an opinion if a promotion could be better since the 5th row in place of the 8th.