This thread seems to have drifted off into the excalibur one.
I agree HIMARS would be good to have, but when considering the limitations of time and money, and if you can only have one, the M777, IMO, is the better choice.
HIMARS might give you range and precision when needed, but even in it's reduced effects form it still carries a large warhead that might not be so desirable in danger close situations or when collateral damage is more likely from the larger blast. Most rockets do not give you much of a sustained fire capability unless you have a good lot of them to make up for the longer reload times, nor does it give you (in most cases) other capabilities for munitions like illumination or multi-spectral smoke.
But the M109, why? Comparing it to the Leopard deploying isn't a very good parallel for a number of reasons, not the least of which because the Leo has stayed in service whereas the M109 was withdrawn, and in any case they both are meant for very different roles.
But if we're going to talk about bringing back some old Artillery gun into service I'd vote for the L5, now there was a honey. She'd stick by you through all kinds of abuse and terrain and still perform for you. You could strip her right down, more so than any other, and get her cleaned up or packed up into all kinds of different transport. A very forgiving gun, mind you if you grabbed one of her legs the wrong way she sure could pinch you bad.
Yes I'm still talking about a gun here, although it might've sounded like I was talking about an old sweetheart, then again, maybe I am.