I would definitely not use to recreate "classic combos", that's way overpowered.
I would suggest another use, however: to bring back the weak and useless spells in rotation.
Put simply (just to give y'all the idea, this hasn't been tested):
You can combine any two spells into one, given that both spells meet the following conditions:
# both spells require Concentration
# both spells are rated red or brown (bad, mediocre or worse) in at least two separate guides
If either or both spells have a better rating than Red, or if you can only find a single guide where it is rated red, the combo fails.
The resulting spell is the same level as the highest of the two original spells, or 3rd level if both are lower.
The resulting spell is one single spell with the effects of both, and obviously requires Concentration.
Use the "worst" value for each spell parameter. The resulting spell takes as long to cast as the slowest of the two original spells, it has the shortest duration and range of the two, etc.
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You have more faith in guides than i do... so i would replace the whole guides mechanic with "subject to review".
if for no other reason than no game or campaign where homebrew stuff is being developed is following "standard assumptions" and generic analysis is barely useful when constrained by "standard assumptions" and practically worthless when not.