Dynasty Warriors: Gundam 3
Robochop.
Defeating key generals results in loot drops, some of which take the form of new Gundam schematics. These can then be created in the workshop between levels. There is a gigantic number of machines on offer in the game, all with different attacks and special moves. As plans come in different tiers of quality, increasing in value as you drill into the game's later missions, so crafting the perfect machine for your play style becomes an obsessive pursuit.
Each mecha comes with a varying number of upgrade slots, allowing you to increase melee effectiveness and so on, and each can be augmented with a handful of pieces of special equipment giving huge customisation options, albeit within fairly narrow parameters (compared to, say, the Armored Core series).
Each machine handles just differently enough to make collecting them all a motivation, and many of the designs are humorous, encouraging you to develop and upgrade outlandish mecha for the fun of it. So it's a crying shame that (as far as we can tell) the online mode doesn't allow you to take modified Gundam into the wild. Rather, a suite of set missions are offered for up to four players to tackle, into which you can take your own levelled pilot but only off-the-shelf mecha, reducing the opportunity for showboating.
It's in the long game where the clichéd criticisms do hit home. While Gundam 3 encourages players to improve their skills over the course of the game, far more emphasis is placed on the economy of external gains. You grow in power primarily by levelling your chosen pilot (thereby unlocking new skills and team-up combination attack opportunities) and by tinkering with your Gundam. Rarely is a new move introduced, and as there are no penalties for lowering the difficulty of a mission, it's possible to approach the game more like FarmVille than a nuanced hack-and-slash action title, devouring levels in search of better 'loot'.
Nevertheless, for a player whose brain is wired the right way, Gundam 3 offers a wide and deep playpen. It deilghts in the moment-by-moment empowerment of piloting a 1970s vintage mecha in battle, the medium-term enjoyment of turning the tide of a battle through strategic thinking, and the long-view 'gotta catch 'em all' collecting loops. If you have never tried a Musou game before, this is where you should start.