The proposal system has been put into place to allow and encourage these sorts of customizations.
Is it?
I thought that the proposal system was to clean up wonky rules (like non-Kalashtar initiating telepathy) and at one point, to allow Dragon magazine elements into the system in a balanced manner.
At one point in time, many elements of Dragon magazine were not allowed here. Only if a Dragon magazine proposal got accepted after debate did something get in. Now, everything from Dragon magazine is allowed.
The judges have already opened the floodgates to all of Dragon magazine. With a proposal to allow player designed custom items, feats, or powers into the game system (abet one at a time), it just introduces an even larger management and balance set of issues.
A custom designed item that is both mace and rod seems strong. The PC does not need to put away the mace in order to pull out the rod. The PC does not have to own two magic items, just one to handle both weapon and implement powers. This seems a bit potent to me. In fact, Unforgettable Cudgel seems potent to me.
As for which classes this item benefits, it obviously benefits Artificers more than most other arcane classes because they would now have an implement / weapon and that do have powers that benefit from it such as Aggravating Force. And many implements do not have a +D8 critical damage increase, only a +D6.
Is this overpowered? Somewhat. It's not overwhelmingly overpowered, but it is extremely potent.
My main issue is not the power of the item, but that of allowing players to design their own items (and eventually feats or powers). Say, for example, that this proposal gets accepted.
Does the DM then get to decide "Well, I know that Tondrek put this item on his wish list, but I don't think it is appropriate."? Or, is it a matter that once such an item is proposed and approved, Tondrek will definitely acquire that item sooner or later?
It's one thing to have WotC wish list items that may or may not be given in the game, it's another to have a proposal item where it's then more or less expected that the PC acquires the item that the player designed.
The players already get to pick their exact starting items. This doesn't happen in all home games. It's a major benefit here. I'm not too keen on the idea that the players also get to design and pick their higher level items as well and design their PCs to the nth degree. That smacks too much of player entitlement. Why exactly do we have a DM if the player designs their own items?
Players designing and picking their magic items, that's over the edge from my perspective. If the item gets accepted, then the player is going to expect to acquire that item come hell or high water. This item expectation will become the norm and players who might not acquire their wish list items from a given DM might start designing their own items, just so that they too get what they want.
And it introduces a management issue on the web pages as to what is allowed and what is not allowed. Not a problem today with just one such item, but what happens when a lot of people start proposing items specifically designed for their PCs? What happens when players start designing their own feats? What happens when Tondrek's item gets accepted, but someone else's item does not?
From my perspective, we shouldn't even go there. We shouldn't even go there for a weak item because it sets a precedence to allow stronger items in later on.
We have an extremely massive list of items, powers, feats, and classes to choose from. Designing your own should be for home games where the DM has the authority to put the brakes on the game element, not in public games where the judges make that decision ahead of time and then it is nearly impossible to reverse later on (unlike WotC who will nerf items). And, it can create issues with haves and have nots. "Where is the 19/20 crit item for my Druid? Oh, I can go design it? Thanks. Where is the Dimension Door spell item for my Fighter? Oh, I can go design it? Thanks.".
Once you open Pandora's Box, it's hard to close it again.