I know exactly what you're talking about. Even though X4 reworked the visuals, the presentation, and even added a melee combat character, the general structure still adhered to the SNES standard (besides Zero lacking a proper weapon cycle, anyway). That was the last "classic formula" X-series game. Each entry since then has, to varying degrees, attempted to re-invent the power-up system (sans Xtreme1, but I don't think much of that title anyway), and we all know that arsenal is a big part of MegaMan.
In this way I think X5 botched up the formula worse than any other; despite the stage select option still existing the game is clearly designed with a predetermined boss order in mind (Grizzly to Red). Most of the power-up distribution was pretty damn ill-conceived, too, being tied to extra boss rewards (getting a high enough status for that in and of itself was ill-conceived, as suicide and pacifism benefit you) that forbade numerous combinations.
X6 recycled the parts while solving the above issue by tying them to stage collection. Despite the obvious rushed production, X6 had a surprisingly deep layout; it was one of the few games where finding stuff was actually engaging, as opposed to the "use weapon x to destroy block y" shpiel that most other games pull. Xtreme2 took a very successful approach, staying as close as possible to the classic formula by shipping the items off to a shop; all the benefits without changing the gameplay focus. X7 would have actually made a nicely replayable game with its New Game Plus chip stacking, if not for the fact that Capcom utterly failed at trying to blend 2D and 3D perspectives (X's whining doesn't help any, either).
X8 owes much of its success to picking the best out of the previous formulas. X7's New Game Plus, Xtreme2's shop/enemy currency, and X6's stage-hidden chips (New Metals) all make their return in some fashion. In addition the X5/X6 double armor layout was revisited and merged with the "equip immediately" joy of the classic games, adding a new mix-and-match aspect. But at the end of the day, it's still a new layout, so it doesn't quite "feel" like a classic X game (although creating a New Game Plus with all chips and no armor parts can help rectify that).
Of course, all sprite style since SNES have worked on the same limitation. The only difference is the amount of crap that can be on screen, the screen size and the use of prerendered images.
And framerate. You can see a drop between MM8 and MM&B, although the fact that Mets don't walk as smoothly wasn't exactly a huge loss.
All in all, it'd give the feel of a SNES game, but the look of a PSX game
I'd just as soon use PS1 sprites and rewrite the physics, to tell you the truth. But who am I kidding? Unless it was an exact deliberate replication of some "classic" game style, sprite reuse on that level would probably get the game eaten alive by reviewers.