I feel like Tunneling doesn't get enough play, so what about something like Tahm Kench (If anyone here has played league of legends) A support unit, (2 Green orbs of course) with fairly low attack, moderate health, and can pick up small units like a vehicle from Red Alert (I guess Starcraft would be a more well known example) or something, then it tunnels, while carrying the other units, then immediately drops them off at target destination.
Or maybe something similar to a Nydus wurm from Starcraft days, as long as you have Point A tunnel and Point B tunnel, you can travel between them, at (maybe not instantly since that would be OP) but an accelerated rate from point A - Point B.
This not only gives tunnel players WAY more accessibility to actually be a viable strategy, but now Tunnel decks don't even NEED something with tunneling, you can just have a normal green deck/splash deck, then 1 or 2 support units that turn the whole deck on its head. (since, I hate to say it, burrowing ritual and the TunnelWurm don't cut it imo)
This also eliminates the problem of each colour having a theme, and not overloading a colour, (since Green already has Root Networks and all) for example, the tunnel thing could just be colourless, or simply by reducing it to a key few units, it doesn't cause for an overloaded theme
This strikes me as an interesting idea, a building that has an effect while it constructs, then when it finishes, could have some secondary effect, although I can't think of anything more for this example, without making it OP