I posted this a year ago on the old forum and I'm pretty sure it's been mentioned a couple of times since then, but it's not listed here and I don't want it to be forgotten in the shuffle.
Basically, what I would like to see is a tempo lock feature for samples. I.E. You tell MPT how many beats/measures or rows to fit the sample into and then it's tempo is automatically locked/adjusted with the tempo of the song. The current version already has a similar feature, but it's not very flexible and it only works well with samples which have an exact BPM. This feature would be especially useful for loops.
Edit: Related entry on the issue tracker: http://bugs.openmpt.org/view.php?id=296
+1
++1