Don't worry about having questions, that's what we set up this thread for! 
Besides, if you've got the question, it's guaranteed someone else does too, so this helps out other customers too!
So the short version is that you won't have any problems with any of the above, but there are a couple things to look for, so here's some more details:
If you don't care about the data in those fields, there's no problem uninstalling the package and thereby deleting those fields from SFDC. The only thing to watch out for is other dependencies within SFDC for things referencing those fields. For example, if you have any reports, workflows, Apex Triggers or 3rd party software (other AppExchange packages) in your SFDC that reference those fields, then they'll need to be updated to remove those references. Pretty sure SFDC won't even let you uninstall the MLM package without first removing those other custom dependencies created elsewhere. We did see this come up recently with other customers, but it was different because the trouble they ran into was a result of the data backfilling into the newly recreated fields, which you won't have, so it should be a moot point here. I created a new doc about it though, so you should check that out just to be sure: Changes to Marketo Salesforce Sync – Preventative Troubleshooting
With the fields removed from SFDC but still remaining on the Marketo side, there is an error to expect, but it's extremely minor. Without recreating the new fields, the corresponding fields in Marketo will essentially be "orphaned" without fields in SFDC to sync to any longer. However, since the new fields weren't recreated in SFDC, the mapping from Marketo will still be pointing to the old SFDC fields that are no longer there. On the next sync cycle, Marketo will try to sync to those fields but will see they are not accessible. On that sync cycle, you will see an error pop up, something to the effect of "unable to sync to record..." (I forget the exact phrasing). But, the reason this is extremely minor is because once Marketo sees that a field is no longer accessible on the SFDC side, it stops trying. So, on the next sync cycle after that, it won't even bother trying to sync to these fields, so there won't be any more errors. If you change your mind down the road and want to recreate the fields to connect to these specific Marketo fields, you'll still be able to do that by creating them with the correct API name like in the doc.
None of this will stop you from being able to create other new fields later on.
So, here's a recap:
1. Go ahead and uninstall the MLM Package to delete the fields in SFDC, but watch out for other things referencing the fields in SFDC that need to be updated.
2. If you don't recreate the new fields, you'll get one error on the next sync cycle and then no other errors at all from then on.
3. If you change your mind later and want to recreate these fields, it'll still reconnect them as long as the API names are set up right.
4. You'll still be able to create other new custom fields later on as needed.
Let me know if that helps!
-Mike
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